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Revolving Credit Facilities (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended
Jul. 31, 2017
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Nov. 15, 2017
Jun. 30, 2017
Dec. 31, 2016
EQM                
Line of Credit Facility                
Line of credit facility, maximum borrowing capacity $ 1,000,000,000           $ 750,000,000  
Revolving credit facility                
Line of Credit Facility                
Line of credit facility, maximum borrowing capacity 1,500,000,000.0              
Amount of borrowings   $ 0   $ 0       $ 0
Letters of credit outstanding under revolving credit facility   0   0       0
Maximum amount of outstanding short-term loans at any time during the period   0 $ 0 0 $ 0      
Revolving credit facility | EQM                
Line of Credit Facility                
Amount of borrowings   105,000,000   105,000,000       0
Additional borrowing capacity $ 500,000,000              
Letters of credit outstanding under revolving credit facility   0   0       $ 0
Maximum amount of outstanding short-term loans at any time during the period   177,000,000 91,000,000 177,000,000 299,000,000      
Average daily balance of short-term loans outstanding during the period   $ 95,000,000 $ 34,000,000 $ 32,000,000 $ 67,000,000      
Weighted average interest rates of average daily balance of short-term loans   2.70% 2.00% 2.70% 1.90%      
Revolving credit facility | Scenario, Forecast                
Line of Credit Facility                
Line of credit facility, maximum borrowing capacity           $ 2,500,000,000.0