Debt - Narrative (Details) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
Secured Debt [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Increase to borrowing capacity	$ 345
Secured Debt [Member] \| Securitization Notes Payable [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 16,300
Weighted average interest rate	2.89%
Senior Notes [Member]
Debt Instrument [Line Items]
Increase in Senior Notes during the period	$ 7,200
Unsecured Debt [Member] \| Commercial Paper [Member]
Debt Instrument [Line Items]
Principal amount outstanding	$ 1,500
Parent Company [Member] \| Senior Notes [Member] \| Fixed Rate Senior Notes [Member]
Debt Instrument [Line Items]
Weighted average interest rate	3.17%

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

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-Section 55
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- Definition

Net increase or decrease in the carrying amount of the debt instrument for the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Section S99
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- Definition

Amount of increase (decrease) of the credit facility.

+ References

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