Benefit Plans - Funded Status of Plan (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 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	1,001
Interest cost	310	307	1,228
Actuarial loss	0		2,474
Benefits paid	(82)		(507)
Projected benefit obligation, end of period	33,348		32,820
Change in Plan Assets
Plan assets at fair value, beginning of period	0	$ 0	0
Company contributions	82		507
Benefits paid	(82)		(507)
Plan assets at fair value, end of period	0		0
Unfunded Status of the Plan	$ (33,348)		$ (32,820)