Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Components of net periodic benefit cost:
Service cost	$ 668	$ 548	$ 614
Interest cost	598	1,107	1,192
Expected return on plan assets	(686)	(1,258)	(1,189)
Plan amendment effect	(3,067)	0	0
Net periodic benefit cost	$ (2,487)	$ 397	$ 617