Fair Value Fair Value (Summary of Fair Value and Carrying Value of Company Financial Instruments) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Estimate of Fair Value Measurement
Assets:
Finance receivables, net	$ 7,021,549	$ 6,921,037
Estimate of Fair Value Measurement \| Secured Debt \| Asset-Backed U.S. Commercial Paper Conduit Facility VIE
Liabilities:
Debt instrument, fair value	279,457	0
Estimate of Fair Value Measurement \| Secured Debt \| Asset-backed Canadian commercial paper conduit facility
Liabilities:
Debt instrument, fair value	174,779	149,338
Estimate of Fair Value Measurement \| Secured Debt \| Term asset-backed securitization debt, VIE
Liabilities:
Debt instrument, fair value	351,767	797,688
Estimate of Fair Value Measurement \| Medium-term Notes
Liabilities:
Debt instrument, fair value	4,189,092	4,139,462
Estimate of Fair Value Measurement \| Senior Notes
Liabilities:
Debt instrument, fair value	784,433	744,552
Estimate of Fair Value Measurement \| Unsecured Commercial Paper
Liabilities:
Unsecured commercial paper	1,273,482	1,055,708
Carrying Value
Assets:
Finance receivables, net	6,965,086	6,835,458
Carrying Value \| Secured Debt \| Asset-Backed U.S. Commercial Paper Conduit Facility VIE
Liabilities:
Debt instrument, fair value	279,457	0
Carrying Value \| Secured Debt \| Asset-backed Canadian commercial paper conduit facility
Liabilities:
Debt instrument, fair value	174,779	149,338
Carrying Value \| Secured Debt \| Term asset-backed securitization debt, VIE
Liabilities:
Debt instrument, fair value	352,624	796,275
Carrying Value \| Medium-term Notes
Liabilities:
Debt instrument, fair value	4,165,706	4,064,940
Carrying Value \| Senior Notes
Liabilities:
Debt instrument, fair value	741,961	741,306
Carrying Value \| Unsecured Commercial Paper
Liabilities:
Unsecured commercial paper	$ 1,273,482	$ 1,055,708

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13476-108611

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Period Type:	instant

- References

No definition available.

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- References

No definition available.

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- Definition

Fair value portion of an agreement for an unconditional promise by the maker to pay the holder a definite sum of money at a future date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.

+ References

No definition available.

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