Financing Agreement - Additional Information (Details) - USD ($)	1 Months Ended
	Dec. 31, 2013	Dec. 31, 2010	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Aug. 31, 2011
Line of Credit Facility [Line Items]
Revolving loan facility, maximum capacity									$ 350,000,000
Letter of credit outstanding			$ 20,339,000
Debt outstanding from dealers			1,422,244,000
Long-term debt			$ 883,000,000		$ 1,112,142,000		$ 425,707,000
Maximum
Line of Credit Facility [Line Items]
Aggregate repurchase obligation			15.00%
Master Notes | Senior Unsecured Notes, 3.81 Percent, Due May 2018
Line of Credit Facility [Line Items]
Issuance of senior notes		$ 25,000,000
Maturity date		May 31, 2018
Master Notes | Senior Unsecured Notes, 4.60 Percent, Due May 2021
Line of Credit Facility [Line Items]
Issuance of senior notes		$ 75,000,000
Maturity date		May 31, 2021
Master Notes | Senior Unsecured Notes 3.13 Percent Due December 2020
Line of Credit Facility [Line Items]
Issuance of senior notes	$ 100,000,000
Maturity date	Dec. 31, 2020
Notes payable and other | Mortgages
Line of Credit Facility [Line Items]
Long-term debt			$ 12,083,000					$ 14,500,000
Long-term Debt
Line of Credit Facility [Line Items]
Revolving loan facility, maximum capacity			750,000,000
Face amount				$ 500,000,000
Long-term line of credit			$ 680,000,000		$ 740,000,000
Revolving loan facility
Line of Credit Facility [Line Items]
Revolving loan facility, maximum capacity				$ 600,000,000		$ 500,000,000