Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Components of Net Periodic Benefit Cost
Service cost	$ 250	$ 300	$ 499	$ 600	$ 1,200
Interest cost	348	310	696	621	$ 1,242
Net loss	53	133	105	266
Net periodic pension cost	$ 651	$ 743	$ 1,300	$ 1,487