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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost |  | $ 5,220,527
Unrealized Gains |  | 130,058
Unrealized Losses |  | (5,358)
Total AFS securities |  | 5,345,227
AFS Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | $ 5,270,798 | 5,076,716
Unrealized Gains | 47,369 | 90,867
Unrealized Losses | (45,067) | (5,061)
Total AFS securities | 5,273,100 | 5,162,522
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 120,092 | 49,326
Unrealized Gains | 1,810 | 647
Unrealized Losses | (592) | (233)
Total AFS securities | 121,310 | 49,740
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 23,202 | 18,040
Unrealized Gains | 36 | 526
Unrealized Losses | (107) | (11)
Total AFS securities | 23,131 | 18,555
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 1,121,615 | 1,539,307
Unrealized Gains | 19,485 | 44,245
Unrealized Losses | (2,631) | (582)
Total AFS securities | 1,138,469 | 1,582,970
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 1,639,852 | 1,588,339
Unrealized Gains | 5,521 | 30,891
Unrealized Losses | (27,965) | (1,762)
Total AFS securities | 1,617,408 | 1,617,468
AFS Fixed Income Securities [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 720,193 | 789,152
Unrealized Gains | 4,112 | 6,508
Unrealized Losses | (6,943) | (202)
Total AFS securities | 717,362 | 795,458
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 527,409 | 382,727
Unrealized Gains | 3,417 | 1,563
Unrealized Losses | (3,748) | (841)
Total AFS securities | 527,078 | 383,449
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 1,118,435 | 709,825
Unrealized Gains | 12,988 | 6,487
Unrealized Losses | (3,081) | (1,430)
Total AFS securities | $ 1,128,342 | 714,882
Equity Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost |  | 143,811
Unrealized Gains |  | 39,191
Unrealized Losses |  | (297)
Total AFS securities |  | 182,705
Equity Securities [Member] | Common Stock [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost |  | 129,696
Unrealized Gains |  | 38,287
Unrealized Losses |  | (226)
Total AFS securities |  | 167,757
Equity Securities [Member] | Nonredeemable Preferred Stock [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost |  | 14,115
Unrealized Gains |  | 904
Unrealized Losses |  | (71)
Total AFS securities |  | $ 14,948

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