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Debt - Additional Information (Detail) - Mplx Lp [Member] - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2017	Jun. 30, 2017	Feb. 10, 2017	Dec. 31, 2016	Nov. 20, 2014
Bank revolving credit facility due 2020
Debt Instrument [Line Items]
Proceeds from Lines of Credit		$ 0
Letters of Credit Outstanding, Amount		3
Line of Credit Facility, Remaining Borrowing Capacity		$ 2,000
Line of Credit Facility, Remaining Borrowing Capacity, Percentage		99.90%
Term loan facility due 2019
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity					$ 250
Debt Instrument, Interest Rate, Effective Percentage		2.377%
Line of Credit [Member] | Bank revolving credit facility due 2020
Debt Instrument [Line Items]
Long-term debt, gross		$ 0		$ 0
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 2,250
Proceeds from (Repayments of) Debt	$ 2,220
Senior Notes [Member] | 4.125% senior notes due 2027
Debt Instrument [Line Items]
Long-term debt, gross		1,250	$ 1,250	0
Debt Instrument, Interest Rate, Stated Percentage			4.125%
Senior Notes [Member] | 5.200% senior notes due 2047
Debt Instrument [Line Items]
Long-term debt, gross		1,000	$ 1,000	$ 0
Debt Instrument, Interest Rate, Stated Percentage			5.20%
Maximum [Member] | Term loan facility due 2019 | Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 250

X

- Definition

Line of Credit Facility, Remaining Borrowing Capacity, Percentage

+ References

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ 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(22))
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- Definition

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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Contractual interest rate for funds borrowed, under the debt agreement.

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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.

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

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- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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-Publisher FASB
-Name Accounting Standards Codification
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The cash inflow from a contractual arrangement with the lender, including letter of credit, standby letter of credit and revolving credit arrangements, under which borrowings can be made up to a specific amount at any point in time with maturities due beyond one year or the operating cycle, if longer.

+ References

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The net cash inflow or outflow in aggregate debt due to repayments and proceeds from additional borrowings.

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