Accumulated Other Comprehensive Income (Loss) - Rollforward of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020
Other Comprehensive Income, Net of Tax [Abstract]
Balance at beginning and end of the period	$ (129,553)		$ 120,093
Unrealized holding gains (losses) on investments arising during the period for which an allowance for expected credit losses has not been recognized	(251,292)	$ (147,369)
Net realized gains (losses) on disposals and non-credit related impairment losses	(1,562)	(624)
Net decrease (increase) in expected credit losses	0	245
Net unrealized gains (losses) on investments	(249,730)	(146,990)
Other adjustments to other comprehensive income (loss)	84	0
Other comprehensive income (loss), net of tax	(249,646)	(146,990)
Other Comprehensive Income
Other Comprehensive Income, before Tax [Abstract]
Balance at beginning of period	152,016	333,829
Unrealized holding gains (losses) on investments arising during the period for which an allowance for expected credit losses has not been recognized	(318,091)	(186,543)
Net realized gains (losses) on disposals and non-credit related impairment losses	(1,978)	(790)
Net decrease (increase) in expected credit losses		310
Net unrealized gains (losses) on investments	(316,113)	(186,063)
Other adjustments to comprehensive income (loss), net	106
Other comprehensive income (loss)	(316,007)	(186,063)
Balance at end of period	(163,991)	147,766
Other Comprehensive Income, Tax [Abstract]
Balance at beginning of period	31,923	70,104
Unrealized holding gains (losses) on investments arising during the period for which an allowance for expected credit losses has not been recognized	(66,799)	(39,174)
Net realized gains (losses) on disposals and non-credit related impairment losses	(416)	(166)
Net decrease (increase) in expected credit losses		65
Net unrealized gains (losses) on investments	(66,383)	(39,073)
Other adjustments to comprehensive income (loss), net	22
Other comprehensive income (loss)	(66,361)	(39,073)
AOCI Tax, Balance at end of period	(34,438)	31,031
Other Comprehensive Income, Net of Tax [Abstract]
Balance at beginning and end of the period	(129,553)	116,735	$ 120,093	$ 263,725
Unrealized holding gains (losses) on investments arising during the period for which an allowance for expected credit losses has not been recognized	(251,292)	(147,369)
Net realized gains (losses) on disposals and non-credit related impairment losses	(1,562)	(624)
Net decrease (increase) in expected credit losses		245
Net unrealized gains (losses) on investments	(249,730)	(146,990)
Other adjustments to other comprehensive income (loss)	84
Other comprehensive income (loss), net of tax	$ (249,646)	$ (146,990)