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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | $ 4,874,648 | $ 4,546,330
Unrealized Gains | 80,702 | 93,777
Unrealized Losses | (16,057) | (24,853)
Total AFS securities | 4,939,293 | 4,615,254
AFS Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 4,753,759 | 4,352,514
Unrealized Gains | 54,178 | 78,504
Unrealized Losses | (15,397) | (22,815)
Total AFS securities | 4,792,540 | 4,408,203
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 75,139 | 99,485
Unrealized Gains | 2,230 | 4,721
Unrealized Losses | (36) | (91)
Total AFS securities | 77,333 | 104,115
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 26,559 | 14,885
Unrealized Gains | 322 | 298
Unrealized Losses | (16) | (2)
Total AFS securities | 26,865 | 15,181
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 1,366,287 | 1,314,779
Unrealized Gains | 18,610 | 44,523
Unrealized Losses | (5,304) | (160)
Total AFS securities | 1,379,593 | 1,359,142
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 1,976,556 | 1,892,296
Unrealized Gains | 27,057 | 23,407
Unrealized Losses | (5,860) | (15,521)
Total AFS securities | 1,997,753 | 1,900,182
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 | 527,876 | 244,541
Unrealized Gains | 1,439 | 531
Unrealized Losses | (355) | (918)
Total AFS securities | 528,960 | 244,154
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 256,356 | 245,252
Unrealized Gains | 1,514 | 750
Unrealized Losses | (1,028) | (2,410)
Total AFS securities | 256,842 | 243,592
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 524,986 | 541,276
Unrealized Gains | 3,006 | 4,274
Unrealized Losses | (2,798) | (3,713)
Total AFS securities | 525,194 | 541,837
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 120,889 | 193,816
Unrealized Gains | 26,524 | 15,273
Unrealized Losses | (660) | (2,038)
Total AFS securities | 146,753 | 207,051
Equity Securities [Member] | Common Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 104,663 | 181,991
Unrealized Gains | 26,250 | 14,796
Unrealized Losses | (305) | (1,998)
Total AFS securities | 130,608 | 194,789
Equity Securities [Member] | Nonredeemable Preferred Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 16,226 | 11,825
Unrealized Gains | 274 | 477
Unrealized Losses | (355) | (40)
Total AFS securities | $ 16,145 | $ 12,262

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