Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,567,285	$ 1,290,550
Other comprehensive income (loss) before reclassifications	144,315	259,289	$ (200,090)
Amounts reclassified from AOCI	(8,876)	(124,364)	12,927
Net current period other comprehensive income (loss)	135,439	134,925	(202,204)
Ending balance	1,790,079	1,567,285	1,290,550
Reclassification adjustment for net investment gains (losses) included in income before income tax	11,235	157,423	(16,363)
Other tax expense (benefit)	2,400	33,100	(3,400)
Cumulative effect of change in accounting principle
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0	0	(15,041)
Ending balance		0	0
Net unrealized investment gains (losses) on securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	230,448	96,941	300,177
Other comprehensive income (loss) before reclassifications	144,713	257,871	(201,122)
Amounts reclassified from AOCI	(8,876)	(124,364)	12,927
Net current period other comprehensive income (loss)	135,837	133,507	(203,236)
Ending balance	366,285	230,448	96,941
Net unrealized investment gains (losses) on securities | Cumulative effect of change in accounting principle
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(15,041)
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(10,767)	(12,185)	(13,217)
Other comprehensive income (loss) before reclassifications	(398)	1,418	1,032
Amounts reclassified from AOCI	0	0	0
Net current period other comprehensive income (loss)	(398)	1,418	1,032
Ending balance	(11,165)	(10,767)	(12,185)
Net Funded Status of Benefit Plans | Cumulative effect of change in accounting principle
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	219,681	84,756	286,960
Ending balance	$ 355,120	$ 219,681	84,756
Total | Cumulative effect of change in accounting principle
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ (15,041)