Other Comprehensive Income (Loss) - Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balances	$ 602,565	$ 605,331	$ 558,393	$ 574,429	$ 649,036	$ 668,459	$ 558,393	$ 668,459
Other Comprehensive Income (Loss) Before Reclassification	(78,761)			(97,945)			(64,143)	(324,168)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0			(18)			(32)	(374)
Total Other Comprehensive Income (Loss)	(78,761)	(18,360)	32,946	(97,963)	(92,694)	(133,885)	(64,175)	(324,542)
Ending Balances	538,392	602,565	605,331	494,701	574,429	649,036	538,392	494,701
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balances	(248,852)	(230,492)	(263,438)	(211,095)	(118,401)	15,484	(263,438)	15,484
Total Other Comprehensive Income (Loss)	(78,761)	(18,360)	32,946	(97,963)	(92,694)	(133,885)
Ending Balances	(327,613)	(248,852)	(230,492)	(309,058)	(211,095)	(118,401)	(327,613)	(309,058)
Unrealized Gains and Losses on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balances	(248,338)		(262,924)	(210,527)		16,052	(262,924)	16,052
Other Comprehensive Income (Loss) Before Reclassification	(78,761)			(97,945)			(64,143)	(324,168)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0			(18)			(32)	(374)
Total Other Comprehensive Income (Loss)	(78,761)			(97,963)			(64,175)	(324,542)
Ending Balances	(327,099)	(248,338)		(308,490)	(210,527)		(327,099)	(308,490)
Postretirement Benefit Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balances	(514)		$ (514)	(568)		$ (568)	(514)	(568)
Other Comprehensive Income (Loss) Before Reclassification	0			0			0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0			0			0	0
Total Other Comprehensive Income (Loss)	0			0			0	0
Ending Balances	$ (514)	$ (514)		$ (568)	$ (568)		$ (514)	$ (568)