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Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months | $ 513,007 | $ 2,026,193
12 Months or Greater | 1,499,842 | 1,554,704
Total | 2,012,849 | 3,580,897
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months | (4,748) | (28,520)
12 Months or Greater | (23,068) | (49,384)
Total | (27,816) | (77,904)
U.S. Treasury securities and obligations of U.S. government corporations and agencies |  |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months | 0 | 23,710
12 Months or Greater | 68,951 | 69,146
Total | 68,951 | 92,856
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months | 0 | (15)
12 Months or Greater | (589) | (1,061)
Total | (589) | (1,076)
Obligations of U.S. states and political subdivisions |  |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months | 17,518 | 316,655
12 Months or Greater | 213,567 | 358,086
Total | 231,085 | 674,741
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months | (444) | (3,875)
12 Months or Greater | (1,782) | (7,110)
Total | (2,226) | (10,985)
Corporate debt securities |  |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months | 149,625 | 1,272,279
12 Months or Greater | 852,102 | 785,627
Total | 1,001,727 | 2,057,906
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months | (1,315) | (18,130)
12 Months or Greater | (8,333) | (22,384)
Total | (9,648) | (40,514)
Asset backed securities (“ABS”) |  |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months | 18,444 | 51,324
12 Months or Greater | 0 | 0
Total | 18,444 | 51,324
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months | (33) | (146)
12 Months or Greater | 0 | 0
Total | (33) | (146)
Residential mortgage backed securities (“RMBS”) |  |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months | 0 | 24
12 Months or Greater | 175,437 | 178,573
Total | 175,437 | 178,597
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months | 0 | 0
12 Months or Greater | (7,859) | (10,309)
Total | (7,859) | (10,309)
Commercial mortgage backed securities (“CMBS”) |  |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months | 9,757 | 65,704
12 Months or Greater | 189,785 | 163,272
Total | 199,542 | 228,976
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months | (94) | (1,060)
12 Months or Greater | (4,505) | (8,520)
Total | (4,599) | (9,580)
Collateralized loan obligations (“CLO”) |  |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Less Than 12 Months | 317,663 | 296,497
12 Months or Greater | 0 | 0
Total | 317,663 | 296,497
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Reportable Items [Abstract] |  |
Less Than 12 Months | (2,862) | (5,294)
12 Months or Greater | 0 | 0
Total | $ (2,862) | $ (5,294)

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