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Summary of Significant Accounting Policies - Components of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | $ (20,257) | $ 12,528 | $ 5,917 | $ 20,155 | $ 125,329 | $ 26,551 | $ 2,261 | $ 15,318 | $ 19,823 | $ 26,923 | $ 11,866 | $ 25,153 | $ 18,343 | $ 169,459 | $ 83,765
Change in net unrealized investment gains (losses) on securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net unrealized investment gains (losses) on securities arising during the period |  |  |  |  |  |  |  |  |  |  |  |  | (275,094) | 105,475 | 6,144
Less: reclassification adjustment for net gains (losses) included in income before income tax |  |  |  |  |  |  |  |  |  |  |  |  | (16,363) | (4,863) | 5,176
Total, before tax |  |  |  |  |  |  |  |  |  |  |  |  | (258,731) | 110,338 | 968
Income tax expense (benefit) |  |  |  |  |  |  |  |  |  |  |  |  | (55,495) | 35,933 | 397
Total, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | (203,236) | 74,405 | 571
Change in net funded status of benefit plans: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Before tax |  |  |  |  |  |  |  |  |  |  |  |  | 1,294 | 1,461 | (37)
Income tax expense (benefit) |  |  |  |  |  |  |  |  |  |  |  |  | 262 | 727 | (14)
Total, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 1,032 | 734 | (23)
Total comprehensive income (loss) |  |  |  |  |  |  |  |  |  |  |  |  | $ (183,861) | $ 244,598 | $ 84,313

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