v3.3.0.814

Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Total		$ 759.1	$ 644.8
Unamortized debt issuance costs(1)	[1]	(4.1)	0.0
Unamortized discount		(1.7)	0.0
Amounts due within one year		(0.8)	(0.8)
Total long-term debt due after one year		752.5	644.0
Capital Lease Obligations [Member] | Marathon Pipe Line Llc [Member]
Debt Instrument [Line Items]
MPL - capital lease obligations due 2020		9.1	9.8
MPLX Revolver [Member] | Line of Credit [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		0.0	385.0
MPLX Term Loan [Member] | Unsecured Debt [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		250.0	250.0
Senior Notes Due Two Thousand Twenty Five [Member] | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		500.0	0.0
Related Party Revolving Credit Agreement [Member] | MPLX Pipe Line Holdings LP [Member]
Debt Instrument [Line Items]
Pipe Line Holdings - revolving credit agreement due 2019		$ 0.0	$ 0.0
[1] (1) We adopted the updated FASB debt issuance cost standard as of June 30, 2015. This has been applied retrospectively and there was no effect to the prior period presented.

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Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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