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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands | Sep. 30, 2016 | Dec. 31, 2015
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | $ 4,805,248 | $ 4,546,330
Unrealized Gains | 177,915 | 93,777
Unrealized Losses | (3,327) | (24,853)
Total AFS securities | 4,979,836 | 4,615,254
AFS Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 4,682,267 | 4,352,514
Unrealized Gains | 152,503 | 78,504
Unrealized Losses | (2,238) | (22,815)
Total AFS securities | 4,832,532 | 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 | 86,491 | 99,485
Unrealized Gains | 3,400 | 4,721
Unrealized Losses | 0 | (91)
Total AFS securities | 89,891 | 104,115
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 9,041 | 14,885
Unrealized Gains | 443 | 298
Unrealized Losses | 0 | (2)
Total AFS securities | 9,484 | 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 | 1,376,245 | 1,314,779
Unrealized Gains | 70,984 | 44,523
Unrealized Losses | (333) | (160)
Total AFS securities | 1,446,896 | 1,359,142
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 2,136,761 | 1,892,296
Unrealized Gains | 60,784 | 23,407
Unrealized Losses | (1,427) | (15,521)
Total AFS securities | 2,196,118 | 1,900,182
AFS Fixed Income Securities [Member] | Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 282,558 | 244,541
Unrealized Gains | 1,964 | 531
Unrealized Losses | (69) | (918)
Total AFS securities | 284,453 | 244,154
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 261,233 | 245,252
Unrealized Gains | 5,634 | 750
Unrealized Losses | (93) | (2,410)
Total AFS securities | 266,774 | 243,592
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 529,938 | 541,276
Unrealized Gains | 9,294 | 4,274
Unrealized Losses | (316) | (3,713)
Total AFS securities | 538,916 | 541,837
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 122,981 | 193,816
Unrealized Gains | 25,412 | 15,273
Unrealized Losses | (1,089) | (2,038)
Total AFS securities | 147,304 | 207,051
Equity Securities [Member] | Common Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 111,746 | 181,991
Unrealized Gains | 24,832 | 14,796
Unrealized Losses | (1,089) | (1,998)
Total AFS securities | 135,489 | 194,789
Equity Securities [Member] | Preferred Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 11,235 | 11,825
Unrealized Gains | 580 | 477
Unrealized Losses | 0 | (40)
Total AFS securities | $ 11,815 | $ 12,262

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