Reclassifications out of Accumulated Other Comprehensive Income - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income, before tax	$ 0	$ 4	$ 0	$ 13
Unrecognized loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income, before tax	(51)	(18)	(152)	(25)
Curtailments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income, before tax	0	46	0	46
Total reclassifications
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income, before tax	(51)	32	(152)	34
Income tax expense	21	(13)	61	(14)
Total reclassifications for the period	$ (30)	$ 19	$ (91)	$ 20