Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net sales									$ 433,947	$ 405,232	$ 383,977
Gross profit	$ 27,795	$ 32,763	$ 27,585	$ 15,390	$ 31,075	$ 35,658	$ 31,678	$ 24,986	103,533	123,397	118,080
Operating Segments | Units
Segment Reporting Information [Line Items]
Net sales									406,331	384,853	363,666
Gross profit									108,214	128,571	120,940
Operating Segments | Parts
Segment Reporting Information [Line Items]
Net sales									28,456	22,050	21,692
Gross profit									13,215	9,377	9,967
Other
Segment Reporting Information [Line Items]
Net sales									(840)	(1,671)	(1,381)
Gross profit									(17,896)	(14,551)	(12,827)
Inter-segment
Segment Reporting Information [Line Items]
Net sales									29,385	29,293	25,406
Gross profit									$ 865	$ 426	$ (105)