Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated other comprehensive income (loss), before tax
Amortization of prior service costs		$ 1,394	$ 1,353
Amortization of net actuarial loss		5,113	12,809
Pension and other postretirement plans, OCI (loss)				$ 10,300
Accumulated other comprehensive income (loss), net of tax
AOCI (loss), beginning of year		(130,284)
Investment securities, OCI (loss)		11,718	(9,937)	(190)
Pension and other postretirement plans, OCI (loss)		1,698	35,712	(8,105)
Reclassification adjustment	[1]			0
OCI (loss)		13,416	25,775	(8,295)
AOCI (loss), end of year		(116,868)	(130,284)
Investment securities:
Accumulated other comprehensive income (loss), before tax
AOCI (loss), beginning of year		(9,169)	3,410	3,954
OCI (loss) before reclassifications - pre TCJA				(648)
OCI (loss) before reclassifications - post TCJA		19,257	(15,372)	1,162
Realized investment (gains) losses		(4,619)	1,297	(1,240)
Impairment losses		195	1,581	182
Cumulative effect of adopting ASU 2016-01, before tax			(85)
Investment securities, OCI (loss)		14,833	(12,579)	(544)
AOCI (loss), end of year		5,664	(9,169)	3,410
Accumulated other comprehensive income (loss), tax
AOCI (loss), beginning of year		(1,926)	716	1,384
OCI (loss) before reclassifications - pre TCJA				(227)
OCI (loss) before reclassifications - post TCJA		4,044	(3,228)	243
Realized investment (gains) losses		(970)	272	(434)
Impairment losses		41	332	64
Cumulative tax effect of adopting ASU 2016-01			(18)
Investment securities, OCI (loss)		3,115	(2,642)	(354)
Reclassification adjustment				(314)
AOCI (loss), end of year		1,189	(1,926)	716
Accumulated other comprehensive income (loss), net of tax
AOCI (loss), beginning of year		(7,243)	2,694	2,570
OCI (loss) before reclassifications - pre TCJA				(421)
OCI (loss) before reclassifications - post TCJA		15,213	(12,144)	919
Realized investment (gains) losses		(3,649)	1,025	(806)
Impairment losses		154	1,249	118
Cumulative effect of adopting ASU 2016-01, net of tax			(67)
Investment securities, OCI (loss)		11,718	(9,937)	(190)
Reclassification adjustment				314
AOCI (loss), end of year		4,475	(7,243)	2,694
Pension and other postretirement plans:
Accumulated other comprehensive income (loss), before tax
AOCI (loss), beginning of year		(155,749)	(200,954)	(190,695)
OCI (loss) before reclassifications		(4,085)	31,401	(20,314)
Amortization of prior service costs		1,394	1,353	871
Amortization of net actuarial loss		4,840	12,451	8,882
Settlement loss				302
Pension and other postretirement plans, OCI (loss)		2,149	45,205	(10,259)
AOCI (loss), end of year		(153,600)	(155,749)	(200,954)
Accumulated other comprehensive income (loss), tax
AOCI (loss), beginning of year		(32,708)	(42,201)	(66,744)
OCI (loss) before reclassifications		(858)	6,594	(7,111)
Amortization of prior service costs		293	284	306
Amortization of net actuarial loss		1,016	2,615	3,109
Settlement loss				106
Impact of change in tax rate				1,436
Pension and other postretirement plans, OCI (loss)		451	9,493	(2,154)
Reclassification adjustment				26,697
AOCI (loss), end of year		(32,257)	(32,708)	(42,201)
Accumulated other comprehensive income (loss), net of tax
AOCI (loss), beginning of year		(123,041)	(158,753)	(123,951)
OCI (loss) before reclassifications		(3,227)	24,807	(13,203)
Amortization of prior service costs		1,101	1,069	565
Amortization of net actuarial loss		3,824	9,836	5,773
Settlement loss				196
Impact of change in tax rate				(1,436)
Pension and other postretirement plans, OCI (loss)		1,698	35,712	(8,105)
Reclassification adjustment				(26,697)
AOCI (loss), end of year		(121,343)	(123,041)	(158,753)
Total
Accumulated other comprehensive income (loss), before tax
AOCI (loss), beginning of year		(164,918)	(197,544)	(186,741)
Investment securities, OCI (loss)		14,833	(12,579)	(544)
Pension and other postretirement plans, OCI (loss)		2,149	45,205	(10,259)
OCI (loss)		16,982	32,626	(10,803)
AOCI (loss), end of year		(147,936)	(164,918)	(197,544)
Accumulated other comprehensive income (loss), tax
AOCI (loss), beginning of year		(34,634)	(41,485)	(65,360)
Investment securities, OCI (loss)		3,115	(2,642)	(354)
Pension and other postretirement plans, OCI (loss)		451	9,493	(2,154)
Reclassification adjustment				26,383
OCI (loss)		3,566	6,851	(2,508)
AOCI (loss), end of year		(31,068)	(34,634)	(41,485)
Accumulated other comprehensive income (loss), net of tax
AOCI (loss), beginning of year		(130,284)	(156,059)	(121,381)
Investment securities, OCI (loss)		11,718	(9,937)	(190)
Pension and other postretirement plans, OCI (loss)		1,698	35,712	(8,105)
Reclassification adjustment	[1]			(26,383)
OCI (loss)		13,416	25,775	(8,295)
AOCI (loss), end of year		$ (116,868)	$ (130,284)	$ (156,059)

[1] A one-time adjustment was made in 2017 to reclassify stranded tax effects of the components of accumulated other comprehensive income ("AOCI") (loss) resulting from enactment of Tax Cuts and Jobs Act ("TCJA") from AOCI (loss) to retained earnings. See Note 2, "Significant Accounting Policies".