Other Comprehensive Income (Loss) - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 181,321	$ 172,521
Ending balance	181,372	174,853
Accumulated Defined Benefit Plans Adjustment Attributable to Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(147)	(102)
Amortization of unrecognized transition obligation	0	0
Change in unamortized net actuarial gain (loss)	0	0
Related deferred taxes (at a Federal Income Tax rate of 21% for March 31, 2018 and 35% for March 31, 2017)	0	0
Ending balance	$ (147)	$ (102)