Summary of Significant Accounting Policies and Other Information - Narrative (Details)	1 Months Ended	3 Months Ended								12 Months Ended
	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) board_member	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) board_member reporting_unit segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)
Concentration Risk [Line Items]
Number of board members | board_member		3								3
Number of board members eligible for election | board_member										4
Number of operating segments | segment										1
Research and development and engineering expenses										$ 26,800,000	$ 18,400,000
Valuation allowance against net deferred tax assets		$ 44,405,000				$ 31,992,000				44,405,000	31,992,000		$ 17,800,000
Deferred tax valuation allowance										9,208,000	2,977,000
Unrecognized tax benefits		1,421,000				1,307,000				1,421,000	1,307,000	$ 1,202,000
Asset impairment charges		$ 2,234,000	$ 0	$ 0	$ 0	$ 1,000	$ 0	$ 0	$ 0	$ 2,234,000	1,000
Number of reporting units | reporting_unit										2
Impairment losses										$ 0	0
Changes in estimates for preexisting warranties					$ 3,800,000					$ 3,842,000	$ 0
Minimum
Concentration Risk [Line Items]
Amortization period										3 years
Maximum
Concentration Risk [Line Items]
Amortization period										15 years
Wells Fargo Revolving Credit Facility
Concentration Risk [Line Items]
Maturity period prior to the final maturity of unsecured senior notes										60 days
Share Purchase Agreement, Weichai Transaction
Concentration Risk [Line Items]
Consideration received on transaction	$ 60,000,000