Segment Reporting (Schedule of Information Relating to Segments) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Net sales to external customers	$ 547,137	$ 554,814	$ 547,483	$ 498,336	$ 402,899	$ 412,370	$ 440,831	$ 422,798	$ 2,147,770	$ 1,678,898	$ 1,403,066
Operating profit (loss)									214,281	200,850	116,254
Total assets	945,858				786,904				945,858	786,904	622,856
Expenditures for long-lived assets									153,445	86,602	67,587
Depreciation and amortization									54,727	46,167	41,624
Purchase of long-lived assets as part of business acquisition									65,000	42,000	38,600
OEM Segment
Segment Reporting Information
Net sales to external customers									1,976,623	1,548,091	1,300,060
Aftermarket Segment
Segment Reporting Information
Net sales to external customers									171,147	130,807	103,006
Corporate and Other
Segment Reporting Information
Operating profit (loss)									0	0	(3,716)
Total assets	83,623				152,308				83,623	152,308	65,439
Depreciation and amortization									$ 314	$ 276	$ 143
Net sales | OEM Segment
Segment Reporting Information
Consolidated risk, percentage									92.00%	92.00%	93.00%
Net sales | Aftermarket Segment
Segment Reporting Information
Consolidated risk, percentage									8.00%	8.00%	7.00%
Customer Concentration Risk | Net sales | Thor Industries, Inc.
Segment Reporting Information
Consolidated risk, percentage									38.00%	37.00%	28.00%
Customer Concentration Risk | Net sales | Berkshire Hathaway Inc.
Segment Reporting Information
Consolidated risk, percentage									25.00%	26.00%	26.00%
Customer Concentration Risk | Net sales | Jayco, Inc.
Segment Reporting Information
Consolidated risk, percentage										10.00%
International | Geographic Concentration Risk | Net sales
Segment Reporting Information
Consolidated risk, percentage									4.00%	2.00%	1.00%
Operating Segments
Segment Reporting Information
Net sales to external customers									$ 2,147,770	$ 1,678,898	$ 1,403,066
Operating profit (loss)									214,281	200,850	119,970
Total assets	862,235				634,596				862,235	634,596	557,417
Expenditures for long-lived assets									153,445	86,602	67,587
Depreciation and amortization									54,413	45,891	41,481
Operating Segments | OEM Segment
Segment Reporting Information
Net sales to external customers									1,976,623	1,548,091	1,300,060
Operating profit (loss)									190,276	180,850	105,224
Total assets	785,926				569,385				785,926	569,385	500,734
Expenditures for long-lived assets									148,570	80,588	65,492
Depreciation and amortization									50,751	42,593	38,583
Operating Segments | Aftermarket Segment
Segment Reporting Information
Net sales to external customers									171,147	130,807	103,006
Operating profit (loss)									24,005	20,000	14,746
Total assets	$ 76,309				$ 65,211				76,309	65,211	56,683
Expenditures for long-lived assets									4,875	6,014	2,095
Depreciation and amortization									$ 3,662	$ 3,298	$ 2,898