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Investments (Available-For-Sale Securities Disclosure) (Details) (USD $) | Mar. 31, 2014 | Dec. 31, 2013
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | $ 3,910,826,000 |  | $ 3,831,327,000 |
Unrealized Gains | 134,079,000 |  | 128,951,000 |
Unrealized Losses | (31,365,000) |  | (51,971,000) |
Fair Value | 4,013,540,000 |  | 3,908,307,000 |
AFS Fixed Income Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 3,748,456,000 |  | 3,675,977,000 |
Unrealized Gains | 98,645,000 |  | 91,434,000 |
Unrealized Losses | (31,248,000) |  | (51,875,000) |
Fair Value | 3,815,853,000 |  | 3,715,536,000 |
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 161,428,000 |  | 163,218,000 |
Unrealized Gains | 10,007,000 |  | 10,661,000 |
Unrealized Losses | (370,000) |  | (504,000) |
Fair Value | 171,065,000 |  | 173,375,000 |
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 31,538,000 |  | 29,781,000 |
Unrealized Gains | 912,000 |  | 906,000 |
Unrealized Losses | (29,000) |  | (72,000) |
Fair Value | 32,421,000 |  | 30,615,000 |
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 963,237,000 |  | 946,455,000 |
Unrealized Gains | 26,617,000 |  | 25,194,000 |
Unrealized Losses | (10,687,000) |  | (20,025,000) |
Fair Value | 979,167,000 |  | 951,624,000 |
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 1,777,186,000 |  | 1,707,928,000 |
Unrealized Gains | 48,305,000 |  | 44,004,000 |
Unrealized Losses | (9,793,000) |  | (17,049,000) |
Fair Value | 1,815,698,000 |  | 1,734,883,000 |
AFS Fixed Income Securities [Member] | Asset-backed Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 131,386,000 |  | 140,430,000 |
Unrealized Gains | 908,000 |  | 934,000 |
Unrealized Losses | (360,000) |  | (468,000) |
Fair Value | 131,934,000 |  | 140,896,000 |
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 172,386,000 | [1] | 172,288,000 | [1]
Unrealized Gains | 4,686,000 | [1] | 2,462,000 | [1]
Unrealized Losses | (2,256,000) | [1] | (3,466,000) | [1]
Fair Value | 174,816,000 | [1] | 171,284,000 | [1]
Government guaranteed agency securities fair value | 24,500,000 |  | 30,000,000 |
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities (RMBS) [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 511,295,000 | [2] | 515,877,000 | [2]
Unrealized Gains | 7,210,000 | [2] | 7,273,000 | [2]
Unrealized Losses | (7,753,000) | [2] | (10,291,000) | [2]
Fair Value | 510,752,000 | [2] | 512,859,000 | [2]
Government guaranteed agency securities fair value | 48,200,000 |  | 55,200,000 |
Equity Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 162,370,000 |  | 155,350,000 |
Unrealized Gains | 35,434,000 |  | 37,517,000 |
Unrealized Losses | (117,000) |  | (96,000) |
Fair Value | $ 197,687,000 |  | $ 192,771,000 |
[1] | CMBS includes government guaranteed agency securities with a fair value of $24.5 million at March 31, 2014 and $30.0 million at December 31, 2013.
[2] | RMBS includes government guaranteed agency securities with a fair value of $48.2 million at March 31, 2014 and $55.2 million at December 31, 2013.

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