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Investments (Details) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) security | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | $ 4,974,616,000 |  | $ 4,946,278,000
Fair Value | 4,930,063,000 |  | 4,983,315,000
Other than Temporary Impairment Losses, Portion in Other Comprehensive Income | 0 |  | $ 0
Proceeds from sales of fixed income securities | 10,844,000 | $ 33,980,000 |
Gross realized gains | 100,000 | 200,000 |
Gross realized losses | $ 300,000 | 300,000 |
Number of securities in unrealized loss position (in securities) | security | 788 |  | 586
Net impairment losses recognized in earnings | $ 0 | $ 0 |
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent) | 102.00% |  |
FHLB advance collateral | $ 165,600,000 |  |
Total fixed income securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 4,974,616,000 |  | $ 4,946,278,000
Gross Unrealized Gains | 31,036,000 |  | 68,584,000
Gross Unrealized Losses | (75,589,000) |  | (31,547,000)
Fair Value | 4,930,063,000 |  | 4,983,315,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 191,018,000 |  | 179,850,000
Gross Unrealized Gains | 256,000 |  | 274,000
Gross Unrealized Losses | (2,212,000) |  | (1,278,000)
Fair Value | 189,062,000 |  | 178,846,000
Obligations of U.S. states and political subdivisions |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 2,093,901,000 |  | 2,105,063,000
Gross Unrealized Gains | 27,926,000 |  | 56,210,000
Gross Unrealized Losses | (19,130,000) |  | (8,749,000)
Fair Value | 2,102,697,000 |  | 2,152,524,000
Corporate debt securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 2,087,977,000 |  | 2,065,475,000
Gross Unrealized Gains | 1,921,000 |  | 10,532,000
Gross Unrealized Losses | (33,819,000) |  | (9,169,000)
Fair Value | 2,056,079,000 |  | 2,066,838,000
Asset backed securities (“ABS”) |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 9,451,000 |  | 4,925,000
Gross Unrealized Gains | 0 |  | 0
Gross Unrealized Losses | (29,000) |  | (2,000)
Fair Value | 9,422,000 |  | 4,923,000
Residential mortgage backed securities (“RMBS”) |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 182,050,000 |  | 189,153,000
Gross Unrealized Gains | 48,000 |  | 60,000
Gross Unrealized Losses | (10,558,000) |  | (7,364,000)
Fair Value | 171,540,000 |  | 181,849,000
Commercial mortgage backed securities (“CMBS”) |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 302,434,000 |  | 301,014,000
Gross Unrealized Gains | 722,000 |  | 1,204,000
Gross Unrealized Losses | (9,800,000) |  | (4,906,000)
Fair Value | 293,356,000 |  | 297,312,000
Collateralized loan obligations (“CLO”) |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 107,785,000 |  | 100,798,000
Gross Unrealized Gains | 163,000 |  | 304,000
Gross Unrealized Losses | (41,000) |  | (79,000)
Fair Value | $ 107,907,000 |  | $ 101,023,000

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