Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Components of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	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, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Net income	$ 47,791	$ 36,474	$ 30,578	$ 18,472	$ 33,001	$ 25,454	$ 93,822	$ 32,166	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 133,315	$ 184,443	$ 18,343
Change in net unrealized investment gains (losses) on fixed maturity securities:
Net unrealized investment gains (losses) on securities arising during the period													183,950	327,363	(275,094)
Less: reclassification adjustment for net investment gains (losses) included in income before income tax													11,235	157,423	(16,363)
Total, before tax													172,715	169,940	(258,731)
Income tax expense (benefit)													36,878	36,433	(55,495)
Total, net of tax													135,837	133,507	(203,236)
Change in net funded status of benefit plans:
Before tax													(503)	1,805	1,294
Income tax expense (benefit)													(105)	387	262
Total, net of tax													(398)	1,418	1,032
Total													$ 268,754	$ 319,368	$ (183,861)