Fair Value Measurements - Valuation of Financial Instruments that are Marked-to-Market on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale investments:
Available-for-sale investments	$ 2	$ 3
Derivative assets	480	565
Derivative liabilities	(735)	(1,701)
Interest Rate Swaps [Member]
Available-for-sale investments:
Derivative assets	392	565
Derivative liabilities	(189)	(208)
Cross-Currency Interest Rate Swaps [Member]
Available-for-sale investments:
Derivative assets	88
Derivative liabilities	(454)	(1,296)
Floor Income Contracts [Member]
Available-for-sale investments:
Derivative liabilities	(74)	(184)
Other [Member]
Available-for-sale investments:
Derivative liabilities	(18)	(13)
Fair Value Measurements Recurring [Member]
Available-for-sale investments:
Available-for-sale investments	2	3
Derivative assets	480	565
Total	482	568
Derivative liabilities	(735)	(1,701)
Total	(735)	(1,701)
Fair Value Measurements Recurring [Member] \| Interest Rate Swaps [Member]
Available-for-sale investments:
Derivative assets	392	565
Derivative liabilities	(189)	(208)
Fair Value Measurements Recurring [Member] \| Cross-Currency Interest Rate Swaps [Member]
Available-for-sale investments:
Derivative assets	88
Derivative liabilities	(454)	(1,296)
Fair Value Measurements Recurring [Member] \| Floor Income Contracts [Member]
Available-for-sale investments:
Derivative liabilities	(74)	(184)
Fair Value Measurements Recurring [Member] \| Other [Member]
Available-for-sale investments:
Derivative liabilities	(18)	(13)
Fair Value Measurements Recurring [Member] \| Agency Residential Mortgage-Backed Securities [Member]
Available-for-sale investments:
Available-for-sale investments		1
Fair Value Measurements Recurring [Member] \| Other [Member]
Available-for-sale investments:
Available-for-sale investments	2	2
Fair Value Measurements Recurring [Member] \| Level 2 [Member]
Available-for-sale investments:
Available-for-sale investments	2	3
Derivative assets	388	553
Total	390	556
Derivative liabilities	(262)	(387)
Total	(262)	(387)
Fair Value Measurements Recurring [Member] \| Level 2 [Member] \| Interest Rate Swaps [Member]
Available-for-sale investments:
Derivative assets	388	553
Derivative liabilities	(144)	(150)
Fair Value Measurements Recurring [Member] \| Level 2 [Member] \| Cross-Currency Interest Rate Swaps [Member]
Available-for-sale investments:
Derivative liabilities	(44)	(53)
Fair Value Measurements Recurring [Member] \| Level 2 [Member] \| Floor Income Contracts [Member]
Available-for-sale investments:
Derivative liabilities	(74)	(184)
Fair Value Measurements Recurring [Member] \| Level 2 [Member] \| Agency Residential Mortgage-Backed Securities [Member]
Available-for-sale investments:
Available-for-sale investments		1
Fair Value Measurements Recurring [Member] \| Level 2 [Member] \| Other [Member]
Available-for-sale investments:
Available-for-sale investments	2	2
Fair Value Measurements Recurring [Member] \| Level 3 [Member]
Available-for-sale investments:
Derivative assets	92	12
Total	92	12
Derivative liabilities	(473)	(1,314)
Total	(473)	(1,314)
Fair Value Measurements Recurring [Member] \| Level 3 [Member] \| Interest Rate Swaps [Member]
Available-for-sale investments:
Derivative assets	4	12
Derivative liabilities	(45)	(58)
Fair Value Measurements Recurring [Member] \| Level 3 [Member] \| Cross-Currency Interest Rate Swaps [Member]
Available-for-sale investments:
Derivative assets	88
Derivative liabilities	(410)	(1,243)
Fair Value Measurements Recurring [Member] \| Level 3 [Member] \| Other [Member]
Available-for-sale investments:
Derivative liabilities	$ (18)	$ (13)

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Fair value of financial and nonfinancial obligations.

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