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Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 5,021,600,000 $ 4,846,800,000
Gross Unrealized Gains 380,600,000 324,600,000
Gross Unrealized Losses (19,500,000) (46,500,000)
Fair Value 5,382,700,000 5,124,900,000
Accrued Expenses and Other Liabilities 453,700,000 453,200,000
Other Receivables 194,300,000 198,600,000
Available-for-sale-Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Securities that have had Recognized Credit Losses in Earnings, Aggregate Losses 0 0
Equity Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Accrued Expenses and Other Liabilities 0  
Other Receivables 0 200,000
Investments in Fixed Maturities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Accrued Expenses and Other Liabilities 5,400,000 100,000
Other Receivables 0 2,700,000
US Government and Government Agencies and Authorities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 542,700,000 321,200,000
Gross Unrealized Gains 19,600,000 22,300,000
Gross Unrealized Losses (6,200,000) (7,200,000)
Fair Value 556,100,000 336,300,000
States and Political Subdivisions [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,595,500,000 1,640,600,000
Gross Unrealized Gains 108,600,000 88,400,000
Gross Unrealized Losses (2,300,000) (14,100,000)
Fair Value 1,701,800,000 1,714,900,000
Foreign Government Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,000,000 3,500,000
Gross Unrealized Gains 200,000 0
Gross Unrealized Losses 0 (100,000)
Fair Value 3,200,000 3,400,000
Corporate Bonds and Notes [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 2,745,800,000 2,758,900,000
Gross Unrealized Gains 245,800,000 209,900,000
Gross Unrealized Losses (11,000,000) (24,000,000)
Fair Value 2,980,600,000 2,944,800,000
Redeemable Preferred Stocks [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 100,000 500,000
Gross Unrealized Gains 0 100,000
Gross Unrealized Losses 0 0
Fair Value 100,000 600,000
Collateralized Loan Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 134,100,000 121,200,000
Gross Unrealized Gains 5,700,000 2,700,000
Gross Unrealized Losses 0 (1,100,000)
Fair Value 139,800,000 122,800,000
Other Mortgage and Asset-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 400,000 900,000
Gross Unrealized Gains 700,000 1,200,000
Gross Unrealized Losses 0 0
Fair Value 1,100,000 2,100,000
Collateralized Debt Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value   900,000
Non-governmental Residential Mortgage-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value $ 1,100,000 $ 1,200,000