Benefit Plans - Funded Status of Plan (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
Change in Projected Benefit Obligation
Projected benefit obligation, beginning of period			$ 32,820	$ 28,624	$ 28,624
Service cost	$ 300	$ 250	600	501	1,001
Interest cost	310	$ 307	621	617	1,228
Actuarial loss			0		2,474
Benefits paid			(228)		(507)
Projected benefit obligation, end of period	33,813		33,813		32,820
Change in Plan Assets
Plan assets at fair value, beginning of period			0	$ 0	0
Company contributions			228		507
Benefits paid			(228)		(507)
Plan assets at fair value, end of period	0		0		0
Unfunded Status of the Plan	$ (33,813)		$ (33,813)		$ (32,820)