Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive Loss by Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 469,577
Other Comprehensive Income (Loss), Net of Tax	109	$ 78
Ending balance	440,464
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(21,814)	(20,149)
Amounts reclassified from accumulated other comprehensive loss	20	(9)
Income tax benefit	(6)	(4)
Net other comprehensive income	26	(5)
Transition adjustment related to adoption of ASU	(2,707)
Ending balance	(24,495)	(20,154)
Accumulated Other-Than-Temporary Impairment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(6,210)	(6,543)
Amounts reclassified from accumulated other comprehensive loss	83	83
Net other comprehensive income	83	83
Ending balance	(6,127)	(6,460)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(28,024)	(26,692)
Amounts reclassified from accumulated other comprehensive loss	103	74
Income tax benefit	(6)	(4)
Net other comprehensive income	109	78
Other Comprehensive Income (Loss), Net of Tax	109	78
Transition adjustment related to adoption of ASU	(2,707)
Ending balance	$ (30,622)	$ (26,614)