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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | $ 5,136,597 | $ 5,220,527
Unrealized Gains | 39,251 | 130,058
Unrealized Losses | (34,211) | (5,358)
AFS securities, fair value | 5,141,637 | 5,345,227
AFS Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost |  | 5,076,716
Unrealized Gains |  | 90,867
Unrealized Losses |  | (5,061)
AFS securities, fair value |  | 5,162,522
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 51,163 | 49,326
Unrealized Gains | 410 | 647
Unrealized Losses | (624) | (233)
AFS securities, fair value | 50,949 | 49,740
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 18,032 | 18,040
Unrealized Gains | 253 | 526
Unrealized Losses | (85) | (11)
AFS securities, fair value | 18,200 | 18,555
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 1,304,321 | 1,539,307
Unrealized Gains | 18,475 | 44,245
Unrealized Losses | (4,636) | (582)
AFS securities, fair value | 1,318,160 | 1,582,970
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 1,624,105 | 1,588,339
Unrealized Gains | 11,033 | 30,891
Unrealized Losses | (13,416) | (1,762)
AFS securities, fair value | 1,621,722 | 1,617,468
AFS Fixed Income Securities [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 789,717 | 789,152
Unrealized Gains | 5,765 | 6,508
Unrealized Losses | (999) | (202)
AFS securities, fair value | 794,483 | 795,458
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 430,894 | 382,727
Unrealized Gains | 404 | 1,563
Unrealized Losses | (4,715) | (841)
AFS securities, fair value | 426,583 | 383,449
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 918,365 | 709,825
Unrealized Gains | 2,911 | 6,487
Unrealized Losses | (9,736) | (1,430)
AFS securities, fair value | $ 911,540 | 714,882
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost |  | 143,811
Unrealized Gains |  | 39,191
Unrealized Losses |  | (297)
AFS securities, fair value |  | 182,705
Equity Securities [Member] | Common Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost |  | 129,696
Unrealized Gains |  | 38,287
Unrealized Losses |  | (226)
AFS securities, fair value |  | 167,757
Equity Securities [Member] | Preferred Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost |  | 14,115
Unrealized Gains |  | 904
Unrealized Losses |  | (71)
AFS securities, fair value |  | $ 14,948

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