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Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Continuous Unrealized Loss Portion, Fair Value [Abstract] |  |
Less Than 12 Months | $ 2,864,747 |
12 Months or Greater | 666,772 |
Total | 3,531,519 |
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract] |  |
Less Than 12 Months | (42,558) |
12 Months or Greater | (33,031) |
Total | (75,589) |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months |  | $ 1,667,170
12 Months or Greater |  | 752,693
Total |  | 2,419,863
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months |  | (9,357)
12 Months or Greater |  | (22,206)
Total |  | (31,563)
U.S. Treasury securities and obligations of U.S. government corporations and agencies |  |
Continuous Unrealized Loss Portion, Fair Value [Abstract] |  |
Less Than 12 Months | 77,510 |
12 Months or Greater | 31,491 |
Total | 109,001 |
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract] |  |
Less Than 12 Months | (1,532) |
12 Months or Greater | (680) |
Total | (2,212) |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months |  | 144,042
12 Months or Greater |  | 31,196
Total |  | 175,238
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months |  | (796)
12 Months or Greater |  | (482)
Total |  | (1,278)
Obligations of U.S. states and political subdivisions |  |
Continuous Unrealized Loss Portion, Fair Value [Abstract] |  |
Less Than 12 Months | 905,755 |
12 Months or Greater | 201,094 |
Total | 1,106,849 |
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract] |  |
Less Than 12 Months | (11,622) |
12 Months or Greater | (7,508) |
Total | (19,130) |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months |  | 505,311
12 Months or Greater |  | 211,684
Total |  | 716,995
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months |  | (3,624)
12 Months or Greater |  | (5,125)
Total |  | (8,749)
Corporate debt securities |  |
Continuous Unrealized Loss Portion, Fair Value [Abstract] |  |
Less Than 12 Months | 1,743,627 |
12 Months or Greater | 148,468 |
Total | 1,892,095 |
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract] |  |
Less Than 12 Months | (26,797) |
12 Months or Greater | (7,022) |
Total | (33,819) |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months |  | 932,350
12 Months or Greater |  | 200,716
Total |  | 1,133,066
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months |  | (4,288)
12 Months or Greater |  | (4,881)
Total |  | (9,169)
Asset backed securities (“ABS”) |  |
Continuous Unrealized Loss Portion, Fair Value [Abstract] |  |
Less Than 12 Months | 9,423 |
12 Months or Greater | 0 |
Total | 9,423 |
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract] |  |
Less Than 12 Months | (29) |
12 Months or Greater | 0 |
Total | (29) |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months |  | 4,923
12 Months or Greater |  | 0
Total |  | 4,923
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months |  | (2)
12 Months or Greater |  | 0
Total |  | (2)
Residential mortgage backed securities (“RMBS”) |  |
Continuous Unrealized Loss Portion, Fair Value [Abstract] |  |
Less Than 12 Months | 14,226 |
12 Months or Greater | 156,842 |
Total | 171,068 |
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract] |  |
Less Than 12 Months | (416) |
12 Months or Greater | (10,142) |
Total | (10,558) |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months |  | 14,979
12 Months or Greater |  | 166,329
Total |  | 181,308
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months |  | (280)
12 Months or Greater |  | (7,084)
Total |  | (7,364)
Commercial mortgage backed securities (“CMBS”) |  |
Continuous Unrealized Loss Portion, Fair Value [Abstract] |  |
Less Than 12 Months | 114,206 |
12 Months or Greater | 126,941 |
Total | 241,147 |
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract] |  |
Less Than 12 Months | (2,162) |
12 Months or Greater | (7,638) |
Total | (9,800) |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months |  | 51,096
12 Months or Greater |  | 138,769
Total |  | 189,865
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months |  | (358)
12 Months or Greater |  | (4,548)
Total |  | (4,906)
Collateralized loan obligations (“CLO”) |  |
Continuous Unrealized Loss Portion, Fair Value [Abstract] |  |
Less Than 12 Months | 0 |
12 Months or Greater | 1,936 |
Total | 1,936 |
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract] |  |
Less Than 12 Months | 0 |
12 Months or Greater | (41) |
Total | $ (41) |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months |  | 14,243
12 Months or Greater |  | 3,568
Total |  | 17,811
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months |  | (7)
12 Months or Greater |  | (72)
Total |  | (79)
Equity securities |  |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months |  | 226
12 Months or Greater |  | 431
Total |  | 657
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months |  | (2)
12 Months or Greater |  | (14)
Total |  | $ (16)

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