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Debt (Credit Agreements) (Detail) $ in Millions |  |  | 12 Months Ended |
--- | --- | --- | --- | ---
Jul. 21, 2017 USD ($) | Oct. 27, 2015 | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Feb. 19, 2017
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt - repayments |  |  | $ 6,780 | $ 416 | $ 1,312 |
MPLX Revolver | MPLX LP |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  | 2,000 |
Term |  | 5 years |  |  |  |
Description of variable rate basis | Adjusted LIBOR or the Alternate Base Rate (as defined in the MPLX Credit Agreement), at our election, plus a specified margin |  |  |  |  |
Number of prior quarterly reporting periods used in determining compliance with covenant of ratio of consolidated net debt to consolidated EBITDA | 4 |  |  |  |  |
MPLX Revolver | MPLX LP | Maximum |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Additional borrowing capacity | $ 500 |  |  |  |  |
Covenant of ratio of consolidated net debt to consolidated EBITDA (in ratio) | 5.0 |  |  |  |  |
Covenant of ratio of consolidated net debt to consolidated EBITDA following certain acquisitions (in ratio) | 5.5 |  |  |  |  |
MPLX Revolver | MPLX LP | Minimum |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Covenant of ratio of consolidated net debt to consolidated EBITDA (in ratio) | 1.0 |  |  |  |  |
Covenant of ratio of consolidated net debt to consolidated EBITDA following certain acquisitions (in ratio) | 1.0 |  |  |  |  |
MPLX Revolver | MPLX LP | Swingline Loan |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Maximum borrowing capacity | $ 100 |  |  |  |  |
Term loan facility due 2019 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  | $ 250 |
Term loan facility due 2019 | MPLX LP |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Interest rate, effective percentage |  |  |  |  |  | 2.407%
MPLX Revolver due July 2022 | MPLX LP |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Maximum borrowing capacity | $ 2,250 |  |  |  |  |
Proceeds from long-term lines of credit |  |  | $ 1,410 | $ 670 |  |
Term | 5 years |  |  |  |  |
Interest rate, effective percentage |  |  | 3.464% | 2.748% |  |
Repayments of long-term lines of credit |  |  | $ 1,915 | $ 165 |  |
Letters of credit outstanding, amount |  |  | 3 | 3 |  |
Remaining borrowing capacity |  |  | $ 2,200 | $ 1,700 |  |
Remaining borrowing capacity, percentage |  |  | 99.90% | 77.40% |  |
MPLX Revolver due July 2022 | MPLX LP | Letter of Credit |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Maximum borrowing capacity | $ 222 |  |  |  |  |
Number of renewals | 2 |  |  |  |  |
Debt Instrument, Renewal Term | 1 year |  |  |  |  |
Line of Credit | MPLX Revolver | MPLX LP |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Proceeds from long-term lines of credit |  |  |  | $ 0 |  |
Line of Credit | MPLX Revolver due July 2022 | MPLX LP |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Expiration date |  |  | Jul. 21, 2022 |  |  |
Long-term debt, outstanding |  |  | $ 0 | $ 505 |  |

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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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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://fasb.org/us-gaap/role/ref/legacyRef -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 4: http://fasb.org/us-gaap/role/ref/legacyRef -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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