Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total postretirement plans	$ 82	$ 111
Income tax expense	(20)	(42)
Total reclassifications for the period, net of tax	62	69
Other income | Prior service cost
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total postretirement plans	113	113
Other income | Actuarial losses
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total postretirement plans	$ (31)	$ (2)