Accumulated Other Comprehensive Income (Loss) - Rollforward of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Before Tax
Unrealized holding gains (losses) on investments arising during the period for which an allowance for expected credit losses has not been recognized	$ (52,056)	$ (256,538)	$ 31,303	$ (574,629)
Net realized gains (losses) on disposals and non-credit related impairment losses	(5,154)	(3,940)	(10,386)	(5,918)
Net unrealized gains (losses) on investments	(46,902)	(252,598)	41,689	(568,711)
Other adjustments to comprehensive income (loss), net	0		227
Other comprehensive income (loss)	(46,902)	(252,598)	41,916	(568,605)
Tax Effect
Unrealized holding gains (losses) on investments arising during the period for which an allowance for expected credit losses has not been recognized	(10,931)	(53,873)	6,574	(120,672)
Net realized gains (losses) on disposals and non-credit related impairment losses	(1,082)	(827)	(2,181)	(1,243)
Net unrealized gains (losses) on investments	(9,849)	(53,046)	8,755	(119,429)
Other adjustments to comprehensive income (loss), net	0		48
Other comprehensive income (loss)	(9,849)	(53,046)	8,803	(119,407)
Net of Tax
Balance at beginning and end of the period	(423,686)		(423,686)			$ (456,799)
Unrealized holding gains (losses) on investments arising during the period for which an allowance for expected credit losses has not been recognized	(41,125)	(202,665)	24,729	(453,957)
Net realized gains (losses) on disposals and non-credit related impairment losses	(4,072)	(3,113)	(8,205)	(4,675)
Net unrealized gains (losses) on investments	(37,053)	(199,552)	32,934	(449,282)
Other adjustments to other comprehensive income (loss)	0	0	179	84
Other comprehensive income (loss), net of tax	(37,053)	(199,552)	33,113	(449,198)
Other Comprehensive Income
Before Tax
Other adjustments to comprehensive income (loss), net			0	106
Tax Effect
Other adjustments to comprehensive income (loss), net			0	22
Net of Tax
Other adjustments to other comprehensive income (loss)			0	84
AOCI Attributable to Parent
Before Tax
Balance at beginning of period	(489,410)	(163,991)	(578,228)	152,016
Balance at end of period	(536,312)	(416,589)	(536,312)	(416,589)
Tax Effect
Balance at beginning of period	(102,777)	(34,438)	(121,429)	31,923
AOCI Tax, Balance at end of period	(112,626)	(87,484)	(112,626)	(87,484)
Net of Tax
Balance at beginning and end of the period	(423,686)	(329,105)	(423,686)	(329,105)	$ (386,633)	$ (456,799)	$ (129,553)	$ 120,093
Net unrealized gains (losses) on investments	$ (37,053)	$ (199,552)	$ 32,934	$ (449,282)