Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Accumulated other comprehensive income (loss), before tax
Investment securities, OCI (loss)	$ 7,314	$ (954)
Pension and other postretirement plans, OCI (loss)	(709)	(1,362)
OCI (loss)	6,605	(2,316)
Accumulated other comprehensive income (loss), tax
Investment securities, OCI (loss)	1,536	(200)
Pension and other postretirement plans, OCI (loss)	(148)	(286)
OCI (loss)	1,388	(486)
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning balance	1,987,258	1,662,835
Investment securities, OCI (loss)	5,778	(754)
Pension and other postretirement plans, OCI (loss)	(561)	(1,076)
OCI (loss)	5,217	(1,830)
Net of tax, ending balance	2,067,351	1,726,180
Total
Accumulated other comprehensive income (loss), before tax
Before tax, beginning balance	(60,244)	(16,963)
Before tax, ending balance	(53,639)	(19,279)
Accumulated other comprehensive income (loss), tax
Tax, beginning balance	12,653	3,563
Tax, ending balance	11,265	4,049
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning balance	(47,591)	(13,400)
OCI (loss)	5,217	(1,830)
Net of tax, ending balance	(42,374)	(15,230)
Investment securities:
Accumulated other comprehensive income (loss), before tax
Before tax, beginning balance	(22,442)	(31,402)
OCI (loss) before reclassifications	6,687	(1,554)
Realized investment losses	262	262
Impairment losses	365	338
Investment securities, OCI (loss)	7,314	(954)
Before tax, ending balance	(15,128)	(32,356)
Accumulated other comprehensive income (loss), tax
Tax, beginning balance	4,714	6,595
OCI (loss) before reclassifications	1,404	(326)
Realized investment losses	55	55
Impairment losses	77	71
Investment securities, OCI (loss)	1,536	(200)
Tax, ending balance	3,178	6,795
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning balance	(17,728)	(24,807)
OCI (loss) before reclassifications	5,283	(1,228)
Realized investment losses	207	207
Impairment losses	288	267
Investment securities, OCI (loss)	5,778	(754)
Net of tax, ending balance	(11,950)	(25,561)
Pension and other postretirement plans:
Accumulated other comprehensive income (loss), before tax
Before tax, beginning balance	(37,802)	14,439
Amortization of prior service costs	422	389
Amortization of net actuarial gain	(654)	(1,751)
Settlement gain	(477)	0
Pension and other postretirement plans, OCI (loss)	(709)	(1,362)
Before tax, ending balance	(38,511)	13,077
Accumulated other comprehensive income (loss), tax
Tax, beginning balance	7,939	(3,032)
Amortization of prior service costs	89	82
Amortization of net actuarial gain	(137)	(368)
Settlement gain	(100)	0
Pension and other postretirement plans, OCI (loss)	(148)	(286)
Tax, ending balance	8,087	(2,746)
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning balance	(29,863)	11,407
Amortization of prior service costs	333	307
Amortization of net actuarial gain	(517)	(1,383)
Settlement gain	(377)	0
Pension and other postretirement plans, OCI (loss)	(561)	(1,076)
Net of tax, ending balance	$ (30,424)	$ 10,331