Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,567,285
Other comprehensive income (loss) before reclassifications	$ 54,312	$ 64,577	97,311	$ 292,043
Amounts reclassified from AOCI	(5,244)	(1,273)	438	(122,603)
Other comprehensive income (loss)	49,068	63,304	97,749	169,440
Ending balance	1,715,678	1,578,612	1,715,678	1,578,612
Pretax reclassification amounts from AOCI, tax	1,400	300	(100)	32,600
Net Unrealized Investment Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	279,129	203,077	230,448	96,941
Other comprehensive income (loss) before reclassifications	54,312	64,577	97,311	292,043
Amounts reclassified from AOCI	(5,244)	(1,273)	438	(122,603)
Other comprehensive income (loss)	49,068	63,304	97,749	169,440
Ending balance	328,197	266,381	328,197	266,381
Pretax reclassification amounts from accumulated other comprehensive income (loss)	6,600	1,600	(600)	155,200
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(10,767)	(12,185)	(10,767)	(12,185)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI	0	0	0	0
Other comprehensive income (loss)	0	0	0	0
Ending balance	(10,767)	(12,185)	(10,767)	(12,185)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	268,362	190,892	219,681	84,756
Ending balance	$ 317,430	$ 254,196	$ 317,430	$ 254,196