Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024
Accumulated other comprehensive income (loss), before tax
Investment securities, OCI (loss)	$ 7,727			$ 25,603			$ 22,967	$ 26,600
Pension and other postretirement plans, OCI (loss)	(1,336)			(1,947)			(2,278)	(7,656)
OCI (loss)	6,391			23,656			20,689	18,944
Accumulated other comprehensive income (loss), tax
Investment securities, OCI (loss)	1,622			5,376			4,823	5,586
Pension and other postretirement plans, OCI (loss)	(280)			(409)			(478)	(1,608)
OCI	1,342			4,967			4,345	3,978
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning balance	2,184,545	$ 2,067,351	$ 1,987,258	1,828,813	$ 1,726,180	$ 1,662,835	1,987,258	1,662,835
Investment securities, OCI (loss)	6,105			20,227			18,144	21,014
Pension and other postretirement plans, OCI (loss)	(1,056)			(1,538)			(1,800)	(6,048)
OCI	5,049	6,078	5,217	18,689	(1,893)	(1,830)	16,344	14,966
Net of tax, ending balance	2,308,878	2,184,545	2,067,351	1,947,955	1,828,813	1,726,180	2,308,878	1,947,955
Total
Accumulated other comprehensive income (loss), before tax
Before tax, beginning balance	(45,946)		(60,244)	(21,675)		(16,963)	(60,244)	(16,963)
Before tax, ending balance	(39,555)	(45,946)		1,981	(21,675)		(39,555)	1,981
Accumulated other comprehensive income (loss), tax
Income Tax, beginning balance	(9,650)		(12,653)	(4,552)		(3,563)	(12,653)	(3,563)
Income Tax, ending balance	(8,308)	(9,650)		415	(4,552)		(8,308)	415
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning balance	(36,296)	(42,374)	(47,591)	(17,123)	(15,230)	(13,400)	(47,591)	(13,400)
OCI	5,049	6,078	5,217	18,689	(1,893)	(1,830)
Net of tax, ending balance	(31,247)	(36,296)	(42,374)	1,566	(17,123)	(15,230)	(31,247)	1,566
Investment securities:
Accumulated other comprehensive income (loss), before tax
Before tax, beginning balance	(7,202)		(22,442)	(30,405)		(31,402)	(22,442)	(31,402)
OCI before reclassifications	7,803			25,586			21,264	23,405
Realized investment gains (losses)	(571)			(142)			122	2,495
Impairment losses	495			159			1,581	700
Investment securities, OCI (loss)	7,727			25,603			22,967	26,600
Before tax, ending balance	525	(7,202)		(4,802)	(30,405)		525	(4,802)
Accumulated other comprehensive income (loss), tax
Income Tax, beginning balance	(1,513)		(4,714)	(6,385)		(6,595)	(4,714)	(6,595)
OCI before reclassifications	1,638			5,373			4,465	4,915
Realized investment gains (losses)	(120)			(30)			26	524
Impairment losses	104			33			332	147
Investment securities, OCI (loss)	1,622			5,376			4,823	5,586
Income Tax, ending balance	109	(1,513)		(1,009)	(6,385)		109	(1,009)
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning balance	(5,689)		(17,728)	(24,020)		(24,807)	(17,728)	(24,807)
OCI before reclassifications	6,165			20,213			16,799	18,490
Realized investment gains (losses)	(451)			(112)			96	1,971
Impairment losses	391			126			1,249	553
Net of tax, ending balance	416	(5,689)		(3,793)	(24,020)		416	(3,793)
Pension and other postretirement plans:
Accumulated other comprehensive income (loss), before tax
Before tax, beginning balance	(38,744)		(37,802)	8,730		14,439	(37,802)	14,439
OCI (loss) before reclassifications	(1,063)			(606)			(1,063)	(2,337)
Amortization of prior service costs	422			413			1,266	1,199
Amortization of net actuarial gain	(633)			(1,695)			(1,942)	(5,180)
Settlement gain	(62)			(59)			(539)	(1,338)
Pension and other postretirement plans, OCI (loss)	(1,336)			(1,947)			(2,278)	(7,656)
Before tax, ending balance	(40,080)	(38,744)		6,783	8,730		(40,080)	6,783
Accumulated other comprehensive income (loss), tax
Income Tax, beginning balance	(8,137)		(7,939)	1,833		3,032	(7,939)	3,032
OCI (loss) before reclassifications	(223)			(127)			(223)	(491)
Amortization of prior service costs	89			87			266	252
Amortization of net actuarial gain	(133)			(356)			(408)	(1,088)
Settlement gain	(13)			(13)			(113)	(281)
Pension and other postretirement plans, OCI (loss)	(280)			(409)			(478)	(1,608)
Income Tax, ending balance	(8,417)	(8,137)		1,424	1,833		(8,417)	1,424
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning balance	(30,607)		$ (29,863)	6,897		$ 11,407	(29,863)	11,407
OCI (loss) before reclassifications	(840)			(479)			(840)	(1,846)
Amortization of prior service costs	333			326			1,000	947
Amortization of net actuarial gain	(500)			(1,339)			(1,534)	(4,092)
Settlement gain	(49)			(46)			(426)	(1,057)
Pension and other postretirement plans, OCI (loss)	(1,056)			(1,538)			(1,800)	(6,048)
Net of tax, ending balance	$ (31,663)	$ (30,607)		$ 5,359	$ 6,897		$ (31,663)	$ 5,359