Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	$ 378,156
Balance	357,495
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(26,692)	$ (22,268)
Amounts reclassified from accumulated other comprehensive loss	4,678	1,609
Income tax (expense) benefit	(1,077)
Net other comprehensive income	3,601	1,609
Balance	(23,091)	(20,659)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(20,149)	(22,268)
Amounts reclassified from accumulated other comprehensive loss	4,429	1,609
Income tax (expense) benefit	(1,077)
Net other comprehensive income	3,352	1,609
Balance	(16,797)	$ (20,659)
Accumulated Other-than-Temporary Impairment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(6,543)
Amounts reclassified from accumulated other comprehensive loss	249
Net other comprehensive income	249
Balance	$ (6,294)