Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Components of Net Periodic Benefit Cost
Service cost	$ 300	$ 250	$ 600	$ 501	$ 1,001
Interest cost	310	307	621	617	$ 1,228
Net loss	133	73	266	145
Net periodic pension cost	$ 743	$ 630	$ 1,487	$ 1,263