Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,567,285
Other comprehensive income (loss) before reclassifications	$ 147,529	$ 107,163	42,999	$ 227,466
Amounts reclassified from AOCI	(5,079)	(114,925)	5,682	(121,330)
Other comprehensive income (loss)	142,450	(7,762)	48,681	106,136
Ending balance	1,639,811	1,499,677	1,639,811	1,499,677
Pretax reclassification amounts from AOCI, tax	1,400	30,500	(1,500)	32,300
Net Unrealized Investment Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	136,679	210,839	230,448	96,941
Other comprehensive income (loss) before reclassifications	147,529	107,163	42,999	227,466
Amounts reclassified from AOCI	(5,079)	(114,925)	5,682	(121,330)
Other comprehensive income (loss)	142,450	(7,762)	48,681	106,136
Ending balance	279,129	203,077	279,129	203,077
Pretax reclassification amounts from accumulated other comprehensive income (loss)	6,400	145,500	(7,200)	153,600
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	125,912	198,654	219,681	84,756
Ending balance	$ 268,362	$ 190,892	$ 268,362	$ 190,892