Retirement Plans, Other Changes in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost	$ (148)	$ (148)	$ (148)
Amortization of actuarial loss (gain)	7,780	16,296	(28,612)
Income tax (expense) benefit	(3,064)	(4,465)	6,767
Other comprehensive income (loss) on pension and postretirement obligations	8,465	12,158	$ (25,025)
RAP
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss)	8,209	17,235
Amortization of prior service cost	(73)	(73)
Amortization of actuarial loss (gain)	3,897	480
Income tax (expense) benefit	(3,040)	(4,532)
Other comprehensive income (loss) on pension and postretirement obligations	8,993	13,109
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss)	169	(188)
Amortization of prior service cost	(75)	(75)
Amortization of actuarial loss (gain)	0	(4)
Income tax (expense) benefit	(23)	67
Other comprehensive income (loss) on pension and postretirement obligations	$ 71	$ (200)