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Debt - Additional Information (Detail) - MPLX LP - USD ($) $ in Millions |  |  |  | 3 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 08, 2018 | Jul. 21, 2017 | Oct. 27, 2015 | Mar. 31, 2018 | Jan. 02, 2018 | Dec. 31, 2017 | Feb. 10, 2017 | Dec. 31, 2016
MPLX Revolver [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  |  |  |  |  |  | $ 2,000
Debt Instrument, Term |  |  | 5 years |  |  |  |  |
Bank revolving credit facility due 2022 |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  | $ 2,250 |  |  |  |  |  |
Debt Instrument, Term |  | 5 years |  |  |  |  |  |
Proceeds from Lines of Credit |  |  |  | $ 50 |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  |  | 2.975% |  |  |  |
Letters of Credit Outstanding |  |  |  | $ 3 |  |  |  |
Line of Credit Facility, Remaining Borrowing Capacity |  |  |  | $ 2,247 |  |  |  |
Line of Credit Facility, Remaining Borrowing Capacity, Percentage |  |  |  | 99.90% |  |  |  |
MPLX 364-Day Term Loan [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Current borrowing capacity |  |  |  |  | $ 4,100 |  |  |
Repayments of Short-term Debt | $ 4,100 |  |  |  |  |  |  |
Line of Credit [Member] | Bank revolving credit facility due 2022 |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt, gross |  |  |  | $ 0 |  | $ 505 |  |
Senior Notes [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt, gross | $ 5,500 |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  |  | $ 2,250 |
Proceeds from new debt |  |  |  | 2,220 |  |  |  |
Senior Notes [Member] | Senior Notes Due March 2023 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt, gross |  |  |  | $ 500 |  | 0 |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  | 3.375% |  |  |  |
Percent of Par | 99.931% |  |  |  |  |  |  |
Senior Notes [Member] | 4.125% senior notes due March 2027 |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt, gross |  |  |  | $ 1,250 |  | 1,250 | $ 1,250 |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  | 4.125% |  |  | 4.125% |
Senior Notes [Member] | 5.200% senior notes due March 2047 |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt, gross |  |  |  | $ 1,000 |  | 1,000 | $ 1,000 |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  | 5.20% |  |  | 5.20% |
Senior Notes [Member] | Senior Notes Due March 2028 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt, gross |  |  |  | $ 1,250 |  | 0 |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  | 4.00% |  |  |  |
Percent of Par | 99.551% |  |  |  |  |  |  |
Senior Notes [Member] | Senior Notes Due April 2038 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt, gross |  |  |  | $ 1,750 |  | 0 |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  | 4.50% |  |  |  |
Percent of Par | 98.811% |  |  |  |  |  |  |
Senior Notes [Member] | Senior Notes Due April 2048 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt, gross |  |  |  | $ 1,500 |  | 0 |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  | 4.70% |  |  |  |
Percent of Par | 99.348% |  |  |  |  |  |  |
Senior Notes [Member] | Senior Notes Due April 2058 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Long-term debt, gross |  |  |  | $ 500 |  | $ 0 |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  | 4.90% |  |  |  |
Percent of Par | 99.289% |  |  |  |  |  |  |

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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)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: 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(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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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 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + 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(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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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 -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 + 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 + Details
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- Definition Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + 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 + Details
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- Definition 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585 + Details
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