Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Components of Net Periodic Benefit Cost
Service cost	$ 250	$ 249	$ 751	$ 746	$ 995
Interest cost	307	289	921	866	$ 1,155
Net loss	73	53	218	160
Net periodic pension cost	$ 630	$ 591	$ 1,890	$ 1,772