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Defined Benefit Pension And Other Postretirement Plans (Summarizes Assumed Health Care Cost Trend Rates) (Detail)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Medical, Pre-65 [Member]
     
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate assumed for the following year 8.00% 8.00% 7.50%
Rate to which the cost trend rate is assumed to decline 5.00% 5.00% 5.00%
Year that the rate reaches the ultimate trend rate 2020 2020 2018
Medical, Post-65 [Member]
     
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate assumed for the following year     7.00% [1]
Rate to which the cost trend rate is assumed to decline     5.00% [1]
Year that the rate reaches the ultimate trend rate     2017 [1]
Medical, Post-65 [Member] | Maximum [Member]
     
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate assumed for the following year 4.00%    
Prescription Drugs [Member]
     
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate assumed for the following year 7.00% 7.00% 7.50%
Rate to which the cost trend rate is assumed to decline 5.00% 5.00% 5.00%
Year that the rate reaches the ultimate trend rate 2018 2018 2018
[1] Effective 2013, as a result of changes in the post-65 medical plan coverage of the Marathon Petroleum Health Plan and the Marathon Petroleum Retiree Health Plan, increases are the lower of the trend rate or 4 percent.