Pension Plans and Other Postretirement Benefits - Components of Net Periodic Cost, Changes in Plan Assets and Benefit Obligations, and AOCI Impacts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amortization of:
Total recognized in other comprehensive income (loss)	$ (1,461)	$ 37	$ (1,815)
Defined Benefit Plan
Service cost:
Benefit accrual	0	0	0
Other expenses	650	650	450
Interest cost	1,091	1,244	1,189
Expected return on plan assets	(1,493)	(1,675)	(1,875)
Settlement loss	0	0	0
Amortization of:
Prior service cost	0	0	0
Actuarial loss	389	393	1,626
Net periodic pension expense	637	612	1,390
Changes in plan assets and benefit obligations included in other comprehensive income (loss):
Prior service cost	0	0	0
Net actuarial loss (gain)	(1,619)	175	930
Amortization of:
Prior service cost	0	0	0
Actuarial loss	(389)	(393)	(1,626)
Total recognized in other comprehensive income (loss)	$ (2,008)	$ (218)	$ (696)
Weighted average assumptions used to determine expense:
Discount rate	3.90%	4.20%	3.66%
Expected return on plan assets	6.25%	6.50%	6.75%
Weighted average assumptions used to determine benefit obligations as of December 31:
Discount rate	3.50%	3.90%	4.20%
Expected return on plan assets	6.25%	6.50%	6.75%
Supplemental Defined Benefit Plans
Service cost:
Benefit accrual	$ 0	$ 0	$ 0
Other expenses	0	0	0
Interest cost	631	687	654
Expected return on plan assets	0	0	0
Settlement loss	0	0	0
Amortization of:
Prior service cost	0	0	0
Actuarial loss	258	233	273
Net periodic pension expense	889	920	927
Changes in plan assets and benefit obligations included in other comprehensive income (loss):
Prior service cost	0	0	0
Net actuarial loss (gain)	805	488	(845)
Amortization of:
Prior service cost	0	0	0
Actuarial loss	(258)	(233)	(273)
Total recognized in other comprehensive income (loss)	$ 547	$ 255	$ (1,118)
Weighted average assumptions used to determine expense:
Discount rate	3.90%	4.20%	3.66%
Weighted average assumptions used to determine benefit obligations as of December 31:
Discount rate	3.50%	3.90%	4.20%