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Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Changes in Level 3 Assets and Liabilities) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Commercial Mortgage Backed Securities [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value $ 3,234      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) (47)      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] (1)      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 1,884      
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 (1,302)      
Commercial Mortgage Backed Securities [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0      
US Government Agencies Debt Securities [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 22,511      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) (340)      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] (96)      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] (1,104)      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 21,053      
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0      
US Government Agencies Debt Securities [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) (110)      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings (110)      
US States and Political Subdivisions Debt Securities [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 41,203      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) 700      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] (35)      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 41,876      
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0      
US States and Political Subdivisions Debt Securities [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 8      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 8      
Foreign Government Debt Securities [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 5,092      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) (88)      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 5,004 $ 12,344    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0      
Foreign Government Debt Securities [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0      
Foreign Government Debt Securities [Member] | Other Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   0    
Funds Withheld At Interest Embedded Derivatives [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 122,194 (22,529)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales [1] 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 135,805 46,173    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3   0    
Funds Withheld At Interest Embedded Derivatives [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   0    
Funds Withheld At Interest Embedded Derivatives [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 13,611 68,702    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 13,611 68,702    
Funds Withheld At Interest Embedded Derivatives [Member] | Interest Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0 0    
Funds Withheld At Interest Embedded Derivatives [Member] | Other Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0 0    
Longevity Swap [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 40,659 26,958    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) 1,085 347    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 44,011 29,170    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3   0    
Longevity Swap [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   0    
Longevity Swap [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0 0    
Longevity Swap [Member] | Interest Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0 0    
Longevity Swap [Member] | Other Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 2,267 1,865    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 2,267 1,865    
Interest Sensitive Contract Liabilities Embedded Derivatives [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value (964,794) (972,930) $ (1,014,229) $ (990,308)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales [1] 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3 0      
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss) 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases [1] (8,508) (6,393)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements [1] 17,807 17,811    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3   0    
Interest Sensitive Contract Liabilities Embedded Derivatives [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings   0    
Interest Sensitive Contract Liabilities Embedded Derivatives [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) 12,901 20,300    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings 14,785 22,362    
Interest Sensitive Contract Liabilities Embedded Derivatives [Member] | Interest Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) (7,544) 34,214    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings (25,351) 16,402    
Interest Sensitive Contract Liabilities Embedded Derivatives [Member] | Other Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings 0 0    
Mortality Swap [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value (1,683) (2,857) $ (1,683) $ (2,462)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3 0      
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss) 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases [1] 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements [1] 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3   0    
Mortality Swap [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings   0    
Mortality Swap [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings 0 0    
Mortality Swap [Member] | Interest Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings 0 0    
Mortality Swap [Member] | Other Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 395    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings 0 (395)    
Equity Securities [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales [1] (28)      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] (48)      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 38,905      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 36,152      
Equity Securities [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0      
Equity Securities [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) (2,705)      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings (2,677)      
Equity Securities [Member] | Interest Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0      
Equity Securities [Member] | Other Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0      
Short-term Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 3,096 3,346    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) (25) 33    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases (146) (32) [1]    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements 0 (135) [1]    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 0      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 3,217 3,276    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3   0    
Short-term Investments [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   0    
Short-term Investments [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0 0    
Short-term Investments [Member] | Interest Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0 0    
Short-term Investments [Member] | Other Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0 0    
Available-for-sale Securities [Member] | Corporate Debt Securities [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 1,337,272 1,272,253    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) (32,852) 4,948    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] (100,170) (45,914)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales [1] (6,180) 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] (75,146) (71,470)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 7,166 13,934    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 1,299,264 1,263,925    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 (30,805) 0    
Available-for-sale Securities [Member] | Corporate Debt Securities [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) (361) (423)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings (361) (423)    
Available-for-sale Securities [Member] | Corporate Debt Securities [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   (1,293)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0 (1,231)    
Available-for-sale Securities [Member] | Canadian Provincial Governments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 593,942 475,965    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) (24,639) 4,525    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales [1] 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 572,747 483,560    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0 0    
Available-for-sale Securities [Member] | Canadian Provincial Governments [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 3,444 3,070    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 3,444 3,070    
Available-for-sale Securities [Member] | Canadian Provincial Governments [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0 0    
Available-for-sale Securities [Member] | Residential mortgage-backed securities        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 107,882 160,291    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) (1,110) 650    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] (20,916) (16,499)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales [1] 0 (10,604)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] (2,963) (6,784)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 0 77    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 120,614 143,430    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 (4,019) (16,819)    
Available-for-sale Securities [Member] | Residential mortgage-backed securities | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) (92) (91)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings (92) (245)    
Available-for-sale Securities [Member] | Residential mortgage-backed securities | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   (346)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0 365    
Available-for-sale Securities [Member] | Asset-backed securities        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 123,474 219,280    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) 853 5,767    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1] (11,000) (10,849)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales [1] 0 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1] (2,739) (18,154)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 0 35,258    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value 130,706 208,436    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 (1,990) (45,582)    
Available-for-sale Securities [Member] | Asset-backed securities | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 106 161    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 106 1,018    
Available-for-sale Securities [Member] | Asset-backed securities | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings $ 2 0    
Available-for-sale Securities [Member] | Commercial Mortgage Backed Securities [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value   21,145    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)   (83)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1]   0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales [1]   (3,720)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1]   (5,401)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3   0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value   1,923    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3   (10,132)    
Available-for-sale Securities [Member] | Commercial Mortgage Backed Securities [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   709    
Available-for-sale Securities [Member] | Commercial Mortgage Backed Securities [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   (595)    
Available-for-sale Securities [Member] | US Treasury and Government [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value   24,488    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)   52    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1]   (104)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales [1]   0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1]   (1,053)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3   0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value   23,474    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3   0    
Available-for-sale Securities [Member] | US Treasury and Government [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   (117)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   (117)    
Available-for-sale Securities [Member] | US Treasury and Government [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   0    
Available-for-sale Securities [Member] | US States and Political Subdivisions Debt Securities [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value   41,666    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)   (843)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1]   0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1]   (33)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value   33,858    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3   (6,844)    
Available-for-sale Securities [Member] | US States and Political Subdivisions Debt Securities [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   (88)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   (88)    
Available-for-sale Securities [Member] | US States and Political Subdivisions Debt Securities [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   0    
Available-for-sale Securities [Member] | US States and Political Subdivisions Debt Securities [Member] | Interest Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   0    
Available-for-sale Securities [Member] | US States and Political Subdivisions Debt Securities [Member] | Other Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   0    
Available-for-sale Securities [Member] | Foreign Government Debt Securities [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value   12,869    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)   (191)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases [1]   0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements [1]   (334)    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3   0    
Available-for-sale Securities [Member] | Foreign Government Debt Securities [Member] | Investment Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   0    
Available-for-sale Securities [Member] | Foreign Government Debt Securities [Member] | Gain (Loss) on Investments [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   0    
Available-for-sale Securities [Member] | Foreign Government Debt Securities [Member] | Interest Income [Member]        
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]        
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)   0    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings   $ 0    
[1] The amount reported within purchases, sales and settlements is the purchase price (for purchases) and the sales/settlement proceeds (for sales and settlements) based upon the actual date purchased or sold/settled. Items purchased and sold/settled in the same period are excluded from the rollforward. The Company had no issuances during the period.