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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands | Jun. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost |  | $ 4,351,361 | $ 4,134,797
Unrealized Gains |  | 90,813 | 133,303
Unrealized Losses |  | (20,473) | (10,578)
Total AFS securities |  | 4,421,701 | 4,257,522
AFS Fixed Income Securities [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost |  | 4,133,223 | 3,975,786
Unrealized Gains |  | 75,631 | 100,578
Unrealized Losses |  | (16,036) | (10,242)
Total AFS securities |  | 4,192,818 | 4,066,122
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost |  | 109,506 | 116,666
Unrealized Gains |  | 6,420 | 7,592
Unrealized Losses |  | (65) | (128)
Total AFS securities |  | 115,861 | 124,130
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost |  | 22,026 | 27,035
Unrealized Gains |  | 693 | 796
Unrealized Losses |  | 0 | 0
Total AFS securities |  | 22,719 | 27,831
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost |  | 1,277,635 | 1,208,776
Unrealized Gains |  | 23,242 | 38,217
Unrealized Losses |  | (4,819) | (729)
Total AFS securities |  | 1,296,058 | 1,246,264
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost |  | 1,794,698 | 1,763,427
Unrealized Gains |  | 36,201 | 42,188
Unrealized Losses |  | (6,324) | (5,809)
Total AFS securities |  | 1,824,575 | 1,799,806
AFS Fixed Income Securities [Member] | Asset-Backed Securities [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost |  | 213,662 | 176,837
Unrealized Gains |  | 990 | 760
Unrealized Losses |  | (155) | (373)
Total AFS securities |  | 214,497 | 177,224
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost | [1] | 213,497 | 177,932
Unrealized Gains | [1] | 1,861 | 2,438
Unrealized Losses | [1] | (1,180) | (777)
Total AFS securities | [1] | 214,178 | 179,593
Government guaranteed agency securities fair value |  | 10,200 | 13,200
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost | [2] | 502,199 | 505,113
Unrealized Gains | [2] | 6,224 | 8,587
Unrealized Losses | [2] | (3,493) | (2,426)
Total AFS securities | [2] | 504,930 | 511,274
Government guaranteed agency securities fair value |  | 25,700 | 32,400
Equity Securities [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost |  | 218,138 | 159,011
Unrealized Gains |  | 15,182 | 32,725
Unrealized Losses |  | (4,437) | (336)
Total AFS securities |  | 228,883 | 191,400
Equity Securities [Member] | Common Stock [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost |  | 200,150 | 159,011
Unrealized Gains |  | 15,159 | 32,725
Unrealized Losses |  | (4,190) | (336)
Total AFS securities |  | 211,119 | $ 191,400
Equity Securities [Member] | Preferred Stock [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Cost/Amortized Cost |  | 17,988 |
Unrealized Gains |  | 23 |
Unrealized Losses |  | (247) |
Total AFS securities |  | $ 17,764 |
[1] | CMBS includes government guaranteed agency securities with a fair value of $10.2 million at June 30, 2015 and $13.2 million at December 31, 2014.
[2] | RMBS includes government guaranteed agency securities with a fair value of $25.7 million at June 30, 2015 and $32.4 million at December 31, 2014.

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