Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Components of Net Periodic Benefit Cost
Service cost	$ 300	$ 250	$ 900	$ 751	$ 1,001
Interest cost	310	307	931	921	$ 1,228
Net loss	133	73	399	218
Net periodic pension cost	$ 743	$ 630	$ 2,230	$ 1,890