Operating Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Leases, Operating [Abstract]
Carrying value of operating lease equipment after depreciation	$ 139,581	$ 118,793
Accumulated depreciation	49,740	42,230
Minimum future lease rental payments due from clients
Total	108,840
2018	28,810
2019	26,880
2020	28,520
2021	14,620
2022	7,050
Thereafter	2,960
Depreciation expense related to operating lease equipment	$ 25,215	$ 21,678	$ 18,280