Employee Benefit Plans - Change in Benefit Obligation and Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Benefit Obligation
Benefit obligation at beginning of year	$ 37,482	$ 36,626	$ 37,076
Service cost	1,508	2,323	1,808
Interest cost	1,097	1,498	1,448
Actuarial (gain) loss	5,156	(2,846)	(2,829)
Benefits paid	(1,204)	(934)	(877)
Curtailments	(4,346)	0	0
Plan amendments	(8,643)	815	0
Benefit obligation at end of year	31,050	37,482	36,626
Change in Plan Assets
Fair value of plan assets at end of year	0	0	0
Funded status at end of year	$ (31,050)	$ (37,482)	$ (36,626)