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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | ---
AFS Fixed Income Securities |  | $ 5,945,360 | $ 6,095,620
Equity Securities |  | 99,989 | 72,937
Short-term investments |  | 517,558 | 282,490
Net Asset Value |  | 39,200 | 37,800
Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 5,945,360 | 6,095,620
Equity Securities |  | 99,989 | 72,937
Short-term investments |  | 517,558 | 282,490
Total assets measured at fair value |  | 6,562,907 | 6,451,047
U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 142,237 | 116,186
Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 12,886 | 18,542
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,197,213 | 1,230,090
Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,909,504 | 1,947,477
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 722,261 | 793,012
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,430,226 | 1,451,969
Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 531,033 | 538,344
Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities | [1] | 98,410 | 69,900
Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities |  | 1,579 | 3,037
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 44,093 | 44,718
Equity Securities |  | 60,754 | 35,182
Short-term investments |  | 504,243 | 265,306
Total assets measured at fair value |  | 609,090 | 345,206
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 44,093 | 41,083
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 3,635
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities | [1] | 59,175 | 32,145
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities |  | 1,579 | 3,037
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 5,837,820 | 6,016,817
Equity Securities |  | 0 | 0
Short-term investments |  | 13,315 | 17,184
Total assets measured at fair value |  | 5,851,135 | 6,034,001
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 98,144 | 75,103
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 12,886 | 18,542
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,194,323 | 1,230,090
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,885,319 | 1,930,426
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 685,889 | 772,343
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,430,226 | 1,451,969
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 531,033 | 538,344
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities | [1] | 0 | 0
Significant Other Observable Inputs (Level 2) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 63,447 | 34,085
Equity Securities |  | 0 | 0
Short-term investments |  | 0 | 0
Total assets measured at fair value |  | 63,447 | 34,085
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 2,890 | 0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 24,185 | 17,051
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 36,372 | 17,034
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities | [1] | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities |  | $ 0 | $ 0
[1] | Investments amounting to $39.2 million at March 31, 2020, and $37.8 million at December 31, 2019, were measured at fair value using net asset value per share (or its practical expedient) and are not classified in the fair value hierarchy. These investments are not redeemable and the timing of liquidations of the underlying assets is unknown at each reporting period disclosed above. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value.

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