Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated other comprehensive income (loss), net of tax
AOCI (loss), beginning of period			$ (78,143)				$ (78,143)
Change in unrealized holding (losses) gains on available-for-sale securities	$ (3,732)			$ 6,143			(9,808)	$ 11,358
Pension and other postretirement plans, OCI	3,463			2,659			10,389	7,979
Total other comprehensive (loss) income, net of tax	(269)	$ 6,139	(5,289)	8,802	$ 40,111	$ (29,576)	581	19,337
AOCI (loss), end of period	(77,562)						(77,562)
Investment securities:
Accumulated other comprehensive income (loss), before tax
AOCI (loss), beginning of period	21,693		29,384	12,265		5,664	29,384	5,664
OCI (loss) before reclassifications	(2,557)			7,792			(8,282)	12,463
Realized investment gains	(2,189)			(137)			(4,069)	(1,107)
Impairment losses	23			122			(63)	3,022
Investment securities, OCI (loss)	(4,723)			7,777			(12,414)	14,378
AOCI (loss), end of period	16,970	21,693		20,042	12,265		16,970	20,042
Accumulated other comprehensive income (loss), tax
AOCI (loss), beginning of period	4,556		6,171	2,575		1,189	6,171	1,189
OCI (loss) before reclassifications	(537)			1,636			(1,739)	2,617
Realized investment gains	(459)			(28)			(854)	(232)
Impairment losses	5			26			(13)	635
Investment securities, OCI (loss)	(991)			1,634			(2,606)	3,020
AOCI (loss), end of period	3,565	4,556		4,209	2,575		3,565	4,209
Accumulated other comprehensive income (loss), net of tax
AOCI (loss), beginning of period	17,137		23,213	9,690		4,475	23,213	4,475
OCI (loss) before reclassifications	(2,020)			6,156			(6,543)	9,846
Realized investment gains	(1,730)			(109)			(3,215)	(875)
Impairment losses	18			96			(50)	2,387
Change in unrealized holding (losses) gains on available-for-sale securities	(3,732)			6,143			(9,808)	11,358
AOCI (loss), end of period	13,405	17,137		15,833	9,690		13,405	15,833
Pension and other postretirement plans:
Accumulated other comprehensive income (loss), before tax
AOCI (loss), beginning of period	(119,533)		(128,300)	(146,866)		(153,600)	(128,300)	(153,600)
Amortization of prior service costs	357			335			1,071	1,007
Amortization of net actuarial loss	4,027			3,031			12,080	9,093
Pension and other postretirement plans, OCI	4,384			3,366			13,151	10,100
AOCI (loss), end of period	(115,149)	(119,533)		(143,500)	(146,866)		(115,149)	(143,500)
Accumulated other comprehensive income (loss), tax
AOCI (loss), beginning of period	(25,103)		(26,944)	(30,843)		(32,257)	(26,944)	(32,257)
Amortization of prior service costs	75			70			225	211
Amortization of net actuarial loss	846			637			2,537	1,910
Pension and other postretirement plans, OCI	921			707			2,762	2,121
AOCI (loss), end of period	(24,182)	(25,103)		(30,136)	(30,843)		(24,182)	(30,136)
Accumulated other comprehensive income (loss), net of tax
AOCI (loss), beginning of period	(94,430)		(101,356)	(116,023)		(121,343)	(101,356)	(121,343)
Amortization of prior service costs	282			265			846	796
Amortization of net actuarial loss	3,181			2,394			9,543	7,183
Pension and other postretirement plans, OCI	3,463			2,659			10,389	7,979
AOCI (loss), end of period	(90,967)	(94,430)		(113,364)	(116,023)		(90,967)	(113,364)
Total
Accumulated other comprehensive income (loss), before tax
AOCI (loss), beginning of period	(97,840)		(98,916)	(134,601)		(147,936)	(98,916)	(147,936)
Investment securities, OCI (loss)	(4,723)			7,777			(12,414)	14,378
Pension and other postretirement plans, OCI	4,384			3,366			13,151	10,100
OCI (loss)	(339)			11,143			737	24,478
AOCI (loss), end of period	(98,179)	(97,840)		(123,458)	(134,601)		(98,179)	(123,458)
Accumulated other comprehensive income (loss), tax
AOCI (loss), beginning of period	(20,547)		(20,773)	(28,268)		(31,068)	(20,773)	(31,068)
Investment securities, OCI (loss)	(991)			1,634			(2,606)	3,020
Pension and other postretirement plans, OCI	921			707			2,762	2,121
OCI (loss)	(70)			2,341			156	5,141
AOCI (loss), end of period	(20,617)	(20,547)		(25,927)	(28,268)		(20,617)	(25,927)
Accumulated other comprehensive income (loss), net of tax
AOCI (loss), beginning of period	(77,293)		(78,143)	(106,333)		(116,868)	(78,143)	(116,868)
Change in unrealized holding (losses) gains on available-for-sale securities	(3,732)			6,143			(9,808)	11,358
Pension and other postretirement plans, OCI	3,463			2,659			10,389	7,979
Total other comprehensive (loss) income, net of tax	(269)	6,139	$ (5,289)	8,802	40,111	$ (29,576)	581	19,337
AOCI (loss), end of period	$ (77,562)	$ (77,293)		$ (97,531)	$ (106,333)		$ (77,562)	$ (97,531)