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Investments (Available-For-Sale Securities Disclosure) (Details) (USD $) | Mar. 31, 2013 | Dec. 31, 2012
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | $ 3,458,553,000 |  | $ 3,263,124,000 |
Unrealized Gains | 186,266,000 |  | 187,105,000 |
Unrealized Losses | (4,093,000) |  | (2,834,000) |
Fair Value | 3,640,726,000 |  | 3,447,395,000 |
AFS Fixed Maturity Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 3,314,989,000 |  | 3,130,683,000 |
Unrealized Gains | 154,644,000 |  | 167,705,000 |
Unrealized Losses | (3,652,000) |  | (2,375,000) |
Fair Value | 3,465,981,000 |  | 3,296,013,000 |
AFS Fixed Maturity Securities [Member] | U.S. Government and Government Agencies [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 229,718,000 |  | 241,874,000 |
Unrealized Gains | 15,950,000 |  | 17,219,000 |
Unrealized Losses | (4,000) |  | (1,000) |
Fair Value | 245,664,000 |  | 259,092,000 |
AFS Fixed Maturity Securities [Member] | Foreign Government [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 28,808,000 |  | 28,813,000 |
Unrealized Gains | 1,472,000 |  | 1,540,000 |
Unrealized Losses | (116,000) |  | (124,000) |
Fair Value | 30,164,000 |  | 30,229,000 |
AFS Fixed Maturity Securities [Member] | Obligations of States and Political Subdivisions [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 810,375,000 |  | 773,953,000 |
Unrealized Gains | 40,630,000 |  | 44,398,000 |
Unrealized Losses | (1,059,000) |  | (327,000) |
Fair Value | 849,946,000 |  | 818,024,000 |
AFS Fixed Maturity Securities [Member] | Corporate Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 1,456,347,000 |  | 1,368,954,000 |
Unrealized Gains | 77,266,000 |  | 81,696,000 |
Unrealized Losses | (860,000) |  | (402,000) |
Fair Value | 1,532,753,000 |  | 1,450,248,000 |
AFS Fixed Maturity Securities [Member] | Asset-backed Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 170,026,000 |  | 126,330,000 |
Unrealized Gains | 1,971,000 |  | 2,319,000 |
Unrealized Losses | (55,000) |  | (9,000) |
Fair Value | 171,942,000 |  | 128,640,000 |
AFS Fixed Maturity Securities [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 144,120,000 | [1] | 133,763,000 | [1]
Unrealized Gains | 3,523,000 | [1] | 4,572,000 | [1]
Unrealized Losses | (1,103,000) | [1] | (1,216,000) | [1]
Fair Value | 146,540,000 | [1] | 137,119,000 | [1]
Government guaranteed agency securities fair value | 42,300,000 |  | 48,900,000 |
AFS Fixed Maturity Securities [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 475,595,000 | [2] | 456,996,000 | [2]
Unrealized Gains | 13,832,000 | [2] | 15,961,000 | [2]
Unrealized Losses | (455,000) | [2] | (296,000) | [2]
Fair Value | 488,972,000 | [2] | 472,661,000 | [2]
Government guaranteed agency securities fair value | 85,000,000 |  | 91,000,000 |
Equity Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 143,564,000 |  | 132,441,000 |
Unrealized Gains | 31,622,000 |  | 19,400,000 |
Unrealized Losses | (441,000) |  | (459,000) |
Fair Value | $ 174,745,000 |  | $ 151,382,000 |
[1] | CMBS includes government guaranteed agency securities with a fair value of $42.3 million at March 31, 2013 and $48.9 million at December 31, 2012.
[2] | RMBS includes government guaranteed agency securities with a fair value of $85.0 million at March 31, 2013 and $91.0 million at December 31, 2012.

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