Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Accumulated other comprehensive income (loss), before tax
Investment securities, OCI	$ 23,950	$ 8,960	$ 35,169
Amortization of prior service costs	1,688	1,611	1,446
Amortization of net actuarial gain	(2,519)	(6,859)	(15,331)
Settlement gain	(572)	(1,338)	0
Pension and other postretirement plans, OCI (loss)	(29,557)	(52,241)	(42,747)
OCI (loss)	(5,607)	(43,281)	(7,578)
Accumulated other comprehensive income (loss), tax
Investment securities, OCI	5,030	1,881	7,385
Pension and other postretirement plans, OCI (loss)	(6,207)	(10,971)	(8,977)
OCI (loss)	(1,177)	(9,090)	(1,592)
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning of year	1,987,258	1,662,835	1,448,408
Investment securities, OCI	18,920	7,079	27,784
Pension and other postretirement plans, OCI (loss)	(23,350)	(41,270)	(33,770)
OCI (loss)	(4,430)	(34,191)	(5,986)
Net of tax, end of year	2,283,374	1,987,258	1,662,835
Total
Accumulated other comprehensive income (loss), before tax
Before Tax, beginning of year	(60,244)	(16,963)	(9,385)
Before Tax, end of year	(65,851)	(60,244)	(16,963)
Accumulated other comprehensive income (loss), tax
Income Tax, beginning of year	(12,653)	(3,563)	(1,971)
Income Tax, end of year	(13,830)	(12,653)	(3,563)
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning of year	(47,591)	(13,400)	(7,414)
OCI (loss)	(4,430)	(34,191)	(5,986)
Net of tax, end of year	(52,021)	(47,591)	(13,400)
Investment securities:
Accumulated other comprehensive income (loss), before tax
Before Tax, beginning of year	(22,442)	(31,402)	(66,571)
OCI before reclassifications	22,119	6,557	26,021
Realized investment (gains) losses	(53)	1,620	6,719
Impairment losses	1,884	783	2,429
Investment securities, OCI	23,950	8,960	35,169
Before Tax, end of year	1,508	(22,442)	(31,402)
Accumulated other comprehensive income (loss), tax
Income Tax, beginning of year	(4,714)	(6,595)	(13,980)
OCI before reclassifications	4,645	1,377	5,464
Realized investment (gains) losses	(11)	340	1,411
Impairment losses	396	164	510
Investment securities, OCI	5,030	1,881	7,385
Income Tax, end of year	316	(4,714)	(6,595)
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning of year	(17,728)	(24,807)	(52,591)
OCI before reclassifications	17,474	5,180	20,557
Realized investment (gains) losses	(42)	1,280	5,308
Impairment losses	1,488	619	1,919
Net of tax, end of year	1,192	(17,728)	(24,807)
Pension and other postretirement plans:
Accumulated other comprehensive income (loss), before tax
Before Tax, beginning of year	(37,802)	14,439	57,186
OCI (loss) before reclassifications	(28,154)	(45,655)	(28,862)
Amortization of prior service costs	1,688	1,611	1,446
Amortization of net actuarial gain	(2,519)	(6,859)	(15,331)
Settlement gain	(572)	(1,338)	0
Pension and other postretirement plans, OCI (loss)	(29,557)	(52,241)	(42,747)
Before Tax, end of year	(67,359)	(37,802)	14,439
Accumulated other comprehensive income (loss), tax
Income Tax, beginning of year	(7,939)	3,032	12,009
OCI (loss) before reclassifications	(5,912)	(9,588)	(6,061)
Amortization of prior service costs	354	338	304
Amortization of net actuarial gain	(529)	(1,440)	(3,220)
Settlement gain	(120)	(281)	0
Pension and other postretirement plans, OCI (loss)	(6,207)	(10,971)	(8,977)
Income Tax, end of year	(14,146)	(7,939)	3,032
Accumulated other comprehensive income (loss), net of tax
Net of tax, beginning of year	(29,863)	11,407	45,177
OCI (loss) before reclassifications	(22,242)	(36,067)	(22,801)
Amortization of prior service costs	1,334	1,273	1,142
Amortization of net actuarial gain	(1,990)	(5,419)	(12,111)
Settlement gain	(452)	(1,057)	0
Pension and other postretirement plans, OCI (loss)	(23,350)	(41,270)	(33,770)
Net of tax, end of year	$ (53,213)	$ (29,863)	$ 11,407