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Accumulated Other Comprehensive Income
 3 Months Ended
Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]  
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]
Accumulated Other Comprehensive Loss

MPLX LP records an accumulated other comprehensive loss on its Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by the LOOP and Explorer joint-interests to their employees. MPLX LP is not a sponsor of these benefit plans. As a transfer between entities under common control, the Partnership recorded the Joint-Interest Acquisition from MPC on its Consolidated Balance Sheets at MPC’s historical basis, which included accumulated other comprehensive loss. MPLX LP’s assumption of the accumulated other comprehensive loss balance had no effect on MPLX LP’s comprehensive income during the period as the balance was accumulated while under the ownership of MPC.

The following table shows the changes in accumulated other comprehensive loss by component during the period December 31, 2017 through March 31, 2018.
(In millions)
Pension
Benefits
 
Other
Post-Retirement Benefits
 
Total
Balance as of December 1, 2017(1)
$
(13
)
 
$
(1
)
 
$
(14
)
Other comprehensive loss - remeasurements(2)
(1
)
 
(1
)
 
(2
)
Balance as of March 31, 2018(1)
$
(14
)

$
(2
)

$
(16
)

(1)
These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included in the Consolidated Statements of Income under the caption “Income (loss) from equity method investments.”
(2)
Components of “Other comprehensive loss - remeasurements” relate to actuarial gains and losses as well as amortization of prior service costs. The Partnership records an adjustment to comprehensive income in accordance with its ownership interest in LOOP and Explorer.