Retirement Plans, Other Changes in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost	$ (73)	$ 50	$ 58
Amortization of actuarial gain	(4,026)	(15,636)	(17,079)
Plan amendments	1,759	0	0
Income tax (expense) benefit	815	5,873	6,507
Other comprehensive loss on pension and postretirement obligations	(1,525)	(9,713)	$ (10,514)
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss)	(9)	19
Amortization of prior service cost	0	0
Amortization of actuarial gain	0	0
Plan amendments	936	0
Income tax (expense) benefit	(353)	(7)
Other comprehensive loss on pension and postretirement obligations	574	12
Retirement Account Plan
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss)	(6,132)	(17,911)
Amortization of prior service cost	(73)	50
Amortization of actuarial gain	2,115	2,256
Plan amendments	823	0
Income tax (expense) benefit	1,168	5,880
Other comprehensive loss on pension and postretirement obligations	$ (2,099)	$ (9,725)