Financing - Additional Information (Detail) ft² in Thousands	1 Months Ended
	Dec. 31, 2013 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Feb. 28, 2015 USD ($)	Jan. 09, 2015 ft²	Dec. 31, 2014 USD ($)	Aug. 31, 2011 USD ($)
Line of Credit Facility [Line Items]
Revolving loan facility, maximum capacity										$ 350,000,000
Letter of credit outstanding			$ 13,368,000
Debt outstanding from dealers			1,438,845,000
Area of Real Estate Property | ft²								725
Long-term debt			$ 1,112,142,000		$ 425,707,000				$ 200,000,000
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 to Banks [Member] | Mortgages [Member]
Line of Credit Facility [Line Items]
Long-term debt							$ 14,500,000
Long-term Debt [Member]
Line of Credit Facility [Line Items]
Revolving loan facility, maximum capacity			$ 750,000,000
Debt Instrument, Face Amount				$ 500,000,000
Long-term Line of Credit			$ 740,000,000		$ 0
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Revolving loan facility, maximum capacity				$ 600,000,000		$ 500,000,000