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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | $ 5,220,527 | $ 4,874,648
Unrealized Gains | 130,058 | 80,702
Unrealized Losses | (5,358) | (16,057)
Total AFS securities | 5,345,227 | 4,939,293
AFS Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 5,076,716 | 4,753,759
Unrealized Gains | 90,867 | 54,178
Unrealized Losses | (5,061) | (15,397)
Total AFS securities | 5,162,522 | 4,792,540
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 49,326 | 75,139
Unrealized Gains | 647 | 2,230
Unrealized Losses | (233) | (36)
Total AFS securities | 49,740 | 77,333
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 18,040 | 26,559
Unrealized Gains | 526 | 322
Unrealized Losses | (11) | (16)
Total AFS securities | 18,555 | 26,865
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 1,539,307 | 1,366,287
Unrealized Gains | 44,245 | 18,610
Unrealized Losses | (582) | (5,304)
Total AFS securities | 1,582,970 | 1,379,593
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 1,588,339 | 1,976,556
Unrealized Gains | 30,891 | 27,057
Unrealized Losses | (1,762) | (5,860)
Total AFS securities | 1,617,468 | 1,997,753
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 | 789,152 | 527,876
Unrealized Gains | 6,508 | 1,439
Unrealized Losses | (202) | (355)
Total AFS securities | 795,458 | 528,960
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 382,727 | 256,356
Unrealized Gains | 1,563 | 1,514
Unrealized Losses | (841) | (1,028)
Total AFS securities | 383,449 | 256,842
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 709,825 | 524,986
Unrealized Gains | 6,487 | 3,006
Unrealized Losses | (1,430) | (2,798)
Total AFS securities | 714,882 | 525,194
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 143,811 | 120,889
Unrealized Gains | 39,191 | 26,524
Unrealized Losses | (297) | (660)
Total AFS securities | 182,705 | 146,753
Equity Securities [Member] | Common Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 129,696 | 104,663
Unrealized Gains | 38,287 | 26,250
Unrealized Losses | (226) | (305)
Total AFS securities | 167,757 | 130,608
Equity Securities [Member] | Nonredeemable Preferred Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 14,115 | 16,226
Unrealized Gains | 904 | 274
Unrealized Losses | (71) | (355)
Total AFS securities | $ 14,948 | $ 16,145

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