Asset-Backed Financing (Narrative) (Detail)	1 Months Ended
	Jun. 30, 2018 CAD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 24, 2017 USD ($)	Jun. 26, 2016 USD ($)
Variable Interest Entity
Company issued secured notes, net of discount and issuance costs		$ 5,714,527,000	$ 5,722,673,000	$ 6,138,192,000
Current unpaid balance - off-balance sheet retail motorcycle finance receivables		146,425,000	93,147,000	165,169,000
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity
Principal balance of finance receivable					$ 301,800,000
Secured Debt
Variable Interest Entity
Company issued secured notes, net of discount and issuance costs		806,860,000	542,936,000	$ 832,271,000
Secured Debt | Asset-Backed U.S. Commercial Paper Conduit Facility VIE | SPEs | U.S. Line of Credit
Variable Interest Entity
Line of credit, maximum borrowing capacity		$ 900,000,000
Debt instrument, term		5 years
Secured Debt | Asset-Backed U.S. Commercial Paper Conduit Facility VIE, Facility Two | SPEs | U.S. Line of Credit
Variable Interest Entity
Line of credit, maximum borrowing capacity		$ 300,000,000.0
Secured Debt | Asset-Backed U.S. Commercial Paper Conduit Facility VIE, Facility One | SPEs | U.S. Line of Credit
Variable Interest Entity
Line of credit, maximum borrowing capacity		$ 600,000,000.0
Secured Debt | Asset-backed Canadian commercial paper conduit facility | Variable Interest Entity, Not Primary Beneficiary | Foreign Line of Credit
Variable Interest Entity
Line of credit, maximum borrowing capacity	$ 220,000,000
Debt instrument, term	5 years
VIE, maximum loss exposure, amount			$ 30,500,000