Retirement Plans, Other Changes in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost	$ (148)	$ (148)	$ (73)
Amortization of actuarial loss (gain)	(28,612)	14,273	(6,141)
Plan amendments	0	0	1,759
Income tax (expense) benefit	6,767	(6,112)	815
Other comprehensive income (loss) on pension and postretirement obligations	(25,025)	10,295	$ (1,525)
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss)	347	(209)
Amortization of prior service cost	(75)	(75)
Amortization of actuarial loss (gain)	8	4
ASU Adjustment RAP (2018-02)	0	0
Income tax (expense) benefit	(71)	107
Other comprehensive income (loss) on pension and postretirement obligations	209	(173)
Retirement Account Plan
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss)	(28,959)	14,482
Amortization of prior service cost	(73)	(73)
Amortization of actuarial loss (gain)	2,195	2,278
ASU Adjustment RAP (2018-02)	(5,235)	0
Income tax (expense) benefit	6,838	(6,219)
Other comprehensive income (loss) on pension and postretirement obligations	$ (25,234)	$ 10,468