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Investments in Unconsolidated Affiliates - Activity Related to Partnership's Investments in Unconsolidated Affiliates (Parenthetical) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Schedule Of Equity Method Investments [Line Items]    
Return of capital from unconsolidated affiliates $ 1.5  
Contributions of property, plant and equipment 16.0 $ 1.0
Little Missouri 4 [Member]    
Schedule Of Equity Method Investments [Line Items]    
Contributions of property, plant and equipment 16.0  
Targa Resources Partners LP [Member]    
Schedule Of Equity Method Investments [Line Items]    
Return of capital from unconsolidated affiliates 1.5  
Targa Resources Partners LP [Member] | Little Missouri 4 [Member]    
Schedule Of Equity Method Investments [Line Items]    
Return of capital from unconsolidated affiliates $ 8.0