Summary of Significant Accounting Policies - Components of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Net income	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 21,040	$ 21,984	$ 16,183	$ 34,275	$ 169,459	$ 83,765	$ 93,482
Change in net unrealized investment gains and losses on fixed maturity and equity securities:
Net unrealized investment gains and losses on fixed maturity and equity securities arising during the period													105,475	6,144	(178,035)
Less: reclassification adjustment for net gains (losses) included in income before income tax													(4,863)	5,176	11,667
Total, before tax													110,338	968	(189,702)
Income tax expense (benefit)													35,933	397	(67,315)
Total, net of tax													74,405	571	(122,387)
Change in net funded status of benefit plan obligations:
Before tax													1,461	(37)	1,815
Income tax expense (benefit)													727	(14)	656
Total, net of tax													734	(23)	1,159
Total comprehensive income (loss)													$ 244,598	$ 84,313	$ (27,746)