Summary of Significant Accounting Policies - Components of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	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