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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | $ 5,216,964 | $ 5,220,527
Unrealized Gains | 25,153 | 130,058
Unrealized Losses | (51,961) | (5,358)
AFS securities, fair value | 5,190,156 | 5,345,227
AFS Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [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] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 97,304 | 49,326
Unrealized Gains | 306 | 647
Unrealized Losses | (1,148) | (233)
AFS securities, fair value | 96,462 | 49,740
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 18,015 | 18,040
Unrealized Gains | 88 | 526
Unrealized Losses | (120) | (11)
AFS securities, fair value | 17,983 | 18,555
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 1,161,322 | 1,539,307
Unrealized Gains | 10,848 | 44,245
Unrealized Losses | (7,261) | (582)
AFS securities, fair value | 1,164,909 | 1,582,970
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 1,662,613 | 1,588,339
Unrealized Gains | 6,342 | 30,891
Unrealized Losses | (19,302) | (1,762)
AFS securities, fair value | 1,649,653 | 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 | 772,738 | 789,152
Unrealized Gains | 4,571 | 6,508
Unrealized Losses | (2,540) | (202)
AFS securities, fair value | 774,769 | 795,458
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 495,208 | 382,727
Unrealized Gains | 328 | 1,563
Unrealized Losses | (5,987) | (841)
AFS securities, fair value | 489,549 | 383,449
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost | 1,009,764 | 709,825
Unrealized Gains | 2,670 | 6,487
Unrealized Losses | (15,603) | (1,430)
AFS securities, fair value | $ 996,831 | 714,882
Equity Securities [Member] |  |
Debt Securities, Available-for-sale [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] |  |
Debt Securities, Available-for-sale [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] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost/Amortized Cost |  | 14,115
Unrealized Gains |  | 904
Unrealized Losses |  | (71)
AFS securities, fair value |  | $ 14,948

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