Summary of Significant Accounting Policies - Components of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Net income	$ 33,001	$ 25,454	$ 93,822	$ 32,166	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 184,443	$ 18,343	$ 169,459
Change in net unrealized investment gains (losses) on securities:
Net unrealized investment gains (losses) on securities arising during the period													327,363	(275,094)	105,475
Less: reclassification adjustment for net investment gains (losses) included in income before income tax													157,423	(16,363)	(4,863)
Total, before tax													169,940	(258,731)	110,338
Income tax expense (benefit)													36,433	(55,495)	35,933
Total, net of tax													133,507	(203,236)	74,405
Change in net funded status of benefit plans:
Before tax													1,805	1,294	1,461
Income tax expense													387	262	727
Total, net of tax													1,418	1,032	734
Total comprehensive income (loss)													$ 319,368	$ (183,861)	$ 244,598