Summary of Significant Accounting Policies - Impairment and Intangibles (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2015
Goodwill and other intangible assets
Number of Reporting Units | item	3
Goodwill	$ 241,006,000	$ 241,006,000
Goodwill impairment	0	0
Finite-lived intangible asset impairment	0	0
Impairment of long-lived assets	0	0
Intangible assets	275,675,000	275,675,000
Gross intangible assets	198,075,000	198,075,000
Accumulated amortization	100,997,000	89,525,000
Dealer network
Goodwill and other intangible assets
Gross intangible assets	80,000,000	80,000,000
Accumulated amortization	$ 59,000,000	55,000,000
Dealer network | Minimum
Goodwill and other intangible assets
Amortization period	15 years
Dealer network | Maximum
Goodwill and other intangible assets
Amortization period	20 years
Trademark
Goodwill and other intangible assets
Gross intangible assets	$ 5,459,000	5,459,000
Accumulated amortization	$ 3,619,000	3,525,000
Trademark | Minimum
Goodwill and other intangible assets
Amortization period	7 years
Trademark | Maximum
Goodwill and other intangible assets
Amortization period	25 years
Patents
Goodwill and other intangible assets
Gross intangible assets	$ 21,136,000	21,136,000
Accumulated amortization	$ 11,974,000	10,721,000
Patents | Minimum
Goodwill and other intangible assets
Amortization period	7 years
Patents | Maximum
Goodwill and other intangible assets
Amortization period	20 years
Customer relations
Goodwill and other intangible assets
Amortization period			15 years
Gross intangible assets	$ 80,920,000	80,920,000
Accumulated amortization	$ 16,607,000	11,304,000
Customer relations | Minimum
Goodwill and other intangible assets
Amortization period	15 years
Customer relations | Maximum
Goodwill and other intangible assets
Amortization period	19 years 6 months
Noncompete agreements
Goodwill and other intangible assets
Amortization period			4 years
Gross intangible assets	$ 8,640,000	8,640,000
Accumulated amortization	$ 7,877,000	7,055,000
Noncompete agreements | Minimum
Goodwill and other intangible assets
Amortization period	4 years
Noncompete agreements | Maximum
Goodwill and other intangible assets
Amortization period	5 years
Work Truck Attachments segment
Goodwill and other intangible assets
Goodwill	$ 160,932,000	160,932,000
Gross intangible assets	195,175,000	195,175,000
Accumulated amortization	88,236,000	81,336,000
Work Truck Solutions
Goodwill and other intangible assets
Goodwill	80,074,000
Gross intangible assets	80,500,000	80,500,000
Accumulated amortization	$ 12,761,000	$ 8,189,000