Pension Plan and Postretirement Benefits Other Than Pension - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan and Postretirement Benefits Other than Pension
Defined benefit plan liabilities		$ 798	$ 15,137
Pension Benefits
Pension Plan and Postretirement Benefits Other than Pension
Defined benefit plan assets		$ 8,471
Defined benefit plan liabilities			$ 13,364	$ 36,392
Other Postretirement Benefits
Pension Plan and Postretirement Benefits Other than Pension
Initial health care cost trend rate prior to age 65 (as a percent)		8.05%	7.02%
Initial health care cost trend rate subsequent to age 65 (as a percent)		9.30%	8.47%
Initial health care cost trend rate (as a percent)				6.82%
Rate to which the cost trend rate is assumed to decline (as a percent)		4.50%
Defined benefit plan liabilities		$ 798	$ 1,773	$ 1,988
Current portion of postretirement liability		$ 250	$ 422	$ 458
SERP
Pension Plan and Postretirement Benefits Other than Pension
Lump sum payment	$ 3,800