Retirement Plans, Other Changes in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost	$ (148)	$ (148)	$ (148)
Amortization of actuarial loss (gain)	16,296	(28,612)	14,273
Income tax (expense) benefit	(4,465)	6,767	(6,112)
Other comprehensive income (loss) on pension and postretirement obligations	12,158	(25,025)	$ 10,295
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss)	17,235	(28,959)
Amortization of prior service cost	(73)	(73)
Amortization of actuarial loss (gain)	480	2,195
ASU Adjustment RAP (2018-02)	0	(5,235)
Income tax (expense) benefit	(4,532)	6,838
Other comprehensive income (loss) on pension and postretirement obligations	13,109	(25,234)
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss)	(188)	347
Amortization of prior service cost	(75)	(75)
Amortization of actuarial loss (gain)	(4)	8
ASU Adjustment RAP (2018-02)	0	0
Income tax (expense) benefit	67	(71)
Other comprehensive income (loss) on pension and postretirement obligations	$ (200)	$ 209