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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | $ 5,172,719 | $ 4,874,648
Unrealized Gains | 139,783 | 80,702
Unrealized Losses | (3,798) | (16,057)
Total AFS securities | 5,308,704 | 4,939,293
AFS Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 5,026,735 | 4,753,759
Unrealized Gains | 109,135 | 54,178
Unrealized Losses | (2,438) | (15,397)
Total AFS securities | 5,133,432 | 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 | 58,820 | 75,139
Unrealized Gains | 906 | 2,230
Unrealized Losses | (137) | (36)
Total AFS securities | 59,589 | 77,333
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 18,149 | 26,559
Unrealized Gains | 650 | 322
Unrealized Losses | 0 | (16)
Total AFS securities | 18,799 | 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 | 1,431,282 | 1,366,287
Unrealized Gains | 49,088 | 18,610
Unrealized Losses | (432) | (5,304)
Total AFS securities | 1,479,938 | 1,379,593
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 1,753,584 | 1,976,556
Unrealized Gains | 41,095 | 27,057
Unrealized Losses | (845) | (5,860)
Total AFS securities | 1,793,834 | 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 | 747,793 | 527,876
Unrealized Gains | 6,305 | 1,439
Unrealized Losses | (143) | (355)
Total AFS securities | 753,955 | 528,960
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 306,173 | 256,356
Unrealized Gains | 2,249 | 1,514
Unrealized Losses | (318) | (1,028)
Total AFS securities | 308,104 | 256,842
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 710,934 | 524,986
Unrealized Gains | 8,842 | 3,006
Unrealized Losses | (563) | (2,798)
Total AFS securities | 719,213 | 525,194
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 145,984 | 120,889
Unrealized Gains | 30,648 | 26,524
Unrealized Losses | (1,360) | (660)
Total AFS securities | 175,272 | 146,753
Equity Securities [Member] | Common Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 131,869 | 104,663
Unrealized Gains | 29,900 | 26,250
Unrealized Losses | (1,342) | (305)
Total AFS securities | 160,427 | 130,608
Equity Securities [Member] | Preferred Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 14,115 | 16,226
Unrealized Gains | 748 | 274
Unrealized Losses | (18) | (355)
Total AFS securities | $ 14,845 | $ 16,145

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