Consolidated Statements of Comprehensive Income (Loss) - 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, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income	$ 148,013	$ 135,103	$ (29,951)	$ 140,461	$ 161,184	$ 173,438	$ 166,730	$ 170,957	$ 393,626	$ 672,309	$ 606,011
Unrealized gains (losses) on investments:
Unrealized holding gains (losses) arising during the period for which an allowance for expected losses has not been recognized									178,761	180,441	(97,356)
Net realized gains (losses) on disposals and non-credit related impairment losses									26,440	8,897	(10,270)
Net decrease (increase) in expected credit losses									(991)	0	0
Net unrealized gains (losses) on investments, net of tax									153,312	171,544	(87,086)
Other adjustments to comprehensive income, net									(75)	(136)	133
Other comprehensive income (loss), net of tax									153,237	171,408	(86,953)
Comprehensive income									$ 546,863	$ 843,717	$ 519,058