Debt - Revolving Credit Facility (Details)	9 Months Ended
Sep. 30, 2018 USD ($) extension
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit	$ 0
Maximum borrowing capacity	$ 3,400,000,000.0
Number of one year extensions \| extension	2
Extension term	1 year
Increase commitment amount (up to)	$ 107,000,000
Covenant, ratio of indebtedness to net capital, maximum	0.65
Ratio of indebtedness to net capital	0.31
Swing-line loans
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 100,000,000
Letter of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 500,000,000

Debt Instrument, Covenant, Ratio Of Indebtedness To Net Capital, Maximum

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- Definition

Line Of Credit Facility, Accordion Feature, Increase Limit

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Line Of Credit Facility, Extension Term

+ References

No definition available.

+ Details

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- Definition

Line of Credit Facility, Number of One Year Extensions

+ References

No definition available.

+ Details

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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.

+ Details

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ 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 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

The carrying value as of the balance sheet date of the current portion 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: 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(19))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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- Definition

Indebtedness divided by net capital.

+ References

No definition available.

+ Details

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- Details

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- Details

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