Retirement Plans, Other Changes in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost	$ (148)	$ (73)	$ 50
Amortization of actuarial loss (gain)	16,555	(4,026)	(15,636)
Plan amendments	0	1,759	0
Income tax (expense) benefit	(6,112)	815	5,873
Other comprehensive income (loss) on pension and postretirement obligations	10,295	(1,525)	$ (9,713)
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss)	(209)	(9)
Amortization of prior service cost	(75)	0
Amortization of actuarial loss (gain)	4	0
Plan amendments	0	936
Income tax (expense) benefit	107	(353)
Other comprehensive income (loss) on pension and postretirement obligations	(173)	574
Retirement Account Plan
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss)	14,482	(6,132)
Amortization of prior service cost	(73)	(73)
Amortization of actuarial loss (gain)	2,278	2,115
Plan amendments	0	823
Income tax (expense) benefit	(6,219)	1,168
Other comprehensive income (loss) on pension and postretirement obligations	$ 10,468	$ (2,099)