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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | $ 5,054,468 | $ 4,874,648
Unrealized Gains | 129,864 | 80,702
Unrealized Losses | (3,916) | (16,057)
Total AFS securities | 5,180,416 | 4,939,293
AFS Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 4,923,005 | 4,753,759
Unrealized Gains | 98,646 | 54,178
Unrealized Losses | (2,929) | (15,397)
Total AFS securities | 5,018,722 | 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 | 66,932 | 75,139
Unrealized Gains | 1,103 | 2,230
Unrealized Losses | (32) | (36)
Total AFS securities | 68,003 | 77,333
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 18,157 | 26,559
Unrealized Gains | 557 | 322
Unrealized Losses | 0 | (16)
Total AFS securities | 18,714 | 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 | 1,360,211 | 1,366,287
Unrealized Gains | 45,641 | 18,610
Unrealized Losses | (1,086) | (5,304)
Total AFS securities | 1,404,766 | 1,379,593
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 1,782,993 | 1,976,556
Unrealized Gains | 39,041 | 27,057
Unrealized Losses | (524) | (5,860)
Total AFS securities | 1,821,510 | 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 | 698,594 | 527,876
Unrealized Gains | 4,108 | 1,439
Unrealized Losses | (178) | (355)
Total AFS securities | 702,524 | 528,960
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 277,320 | 256,356
Unrealized Gains | 2,128 | 1,514
Unrealized Losses | (243) | (1,028)
Total AFS securities | 279,205 | 256,842
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 718,798 | 524,986
Unrealized Gains | 6,068 | 3,006
Unrealized Losses | (866) | (2,798)
Total AFS securities | 724,000 | 525,194
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 131,463 | 120,889
Unrealized Gains | 31,218 | 26,524
Unrealized Losses | (987) | (660)
Total AFS securities | 161,694 | 146,753
Equity Securities [Member] | Common Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 115,315 | 104,663
Unrealized Gains | 30,108 | 26,250
Unrealized Losses | (987) | (305)
Total AFS securities | 144,436 | 130,608
Equity Securities [Member] | Preferred Stock [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost/Amortized Cost | 16,148 | 16,226
Unrealized Gains | 1,110 | 274
Unrealized Losses | 0 | (355)
Total AFS securities | $ 17,258 | $ 16,145

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