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Investments (Available-For-Sale Securities Disclosure) (Details) (USD $) | Sep. 30, 2014 | Dec. 31, 2013
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | $ 4,064,671,000 |  | $ 3,831,327,000 |
Unrealized Gains | 133,475,000 |  | 128,951,000 |
Unrealized Losses | (19,242,000) |  | (51,971,000) |
Total AFS securities | 4,178,904,000 |  | 3,908,307,000 |
AFS Fixed Income Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 3,884,252,000 |  | 3,675,977,000 |
Unrealized Gains | 98,678,000 |  | 91,434,000 |
Unrealized Losses | (15,292,000) |  | (51,875,000) |
Total AFS securities | 3,967,638,000 |  | 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 | 143,742,000 |  | 163,218,000 |
Unrealized Gains | 8,210,000 |  | 10,661,000 |
Unrealized Losses | (263,000) |  | (504,000) |
Total AFS securities | 151,689,000 |  | 173,375,000 |
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 27,041,000 |  | 29,781,000 |
Unrealized Gains | 832,000 |  | 906,000 |
Unrealized Losses | (6,000) |  | (72,000) |
Total AFS securities | 27,867,000 |  | 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 | 1,196,478,000 |  | 946,455,000 |
Unrealized Gains | 34,533,000 |  | 25,194,000 |
Unrealized Losses | (1,777,000) |  | (20,025,000) |
Total AFS securities | 1,229,234,000 |  | 951,624,000 |
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 1,725,290,000 |  | 1,707,928,000 |
Unrealized Gains | 44,571,000 |  | 44,004,000 |
Unrealized Losses | (6,631,000) |  | (17,049,000) |
Total AFS securities | 1,763,230,000 |  | 1,734,883,000 |
AFS Fixed Income Securities [Member] | Asset-Backed Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 146,557,000 |  | 140,430,000 |
Unrealized Gains | 791,000 |  | 934,000 |
Unrealized Losses | (382,000) |  | (468,000) |
Total AFS securities | 146,966,000 |  | 140,896,000 |
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 164,934,000 | [1] | 172,288,000 | [1]
Unrealized Gains | 2,077,000 | [1] | 2,462,000 | [1]
Unrealized Losses | (1,493,000) | [1] | (3,466,000) | [1]
Total AFS securities | 165,518,000 | [1] | 171,284,000 | [1]
Government guaranteed agency securities fair value | 17,600,000 |  | 30,000,000 |
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 480,210,000 | [2] | 515,877,000 | [2]
Unrealized Gains | 7,664,000 | [2] | 7,273,000 | [2]
Unrealized Losses | (4,740,000) | [2] | (10,291,000) | [2]
Total AFS securities | 483,134,000 | [2] | 512,859,000 | [2]
Government guaranteed agency securities fair value | 35,900,000 |  | 55,200,000 |
Equity Securities [Member] |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Cost/Amortized Cost | 180,419,000 |  | 155,350,000 |
Unrealized Gains | 34,797,000 |  | 37,517,000 |
Unrealized Losses | (3,950,000) |  | (96,000) |
Total AFS securities | $ 211,266,000 |  | $ 192,771,000 |
[1] | CMBS includes government guaranteed agency securities with a fair value of $17.6 million at September 30, 2014 and $30.0 million at December 31, 2013.
[2] | RMBS includes government guaranteed agency securities with a fair value of $35.9 million at September 30, 2014 and $55.2 million at December 31, 2013.

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