Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax	$ (1,728)	$ (631)
Period opening balance	(43,983)
Net current period other comprehensive income ending balance	1,419	401
Period closing balance	(55,416)		$ (43,983)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax	(309)	(230)	(230)
Other comprehensive income (loss), tax	500	300
Reclassification of the income tax effects of the Tax Cuts and Jobs Act	(12,852)
Defined Benefit Pension Plan Adjustments
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Period opening balance	(43,983)	(51,753)	(51,753)
Net current period other comprehensive income ending balance	1,419	401
Period closing balance	$ (55,416)	$ (51,352)	$ (43,983)