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Investments in Unconsolidated Affiliates - Activity Related to Partnership's Investments in Unconsolidated Affiliates (Parenthetical) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Feb. 06, 2018
Schedule Of Equity Method Investments [Line Items] |  |  |  |
Return of capital from unconsolidated affiliate | $ 13.2 | $ 3.5 | $ 5.5 |
Contribution of property, plant and equipment to investment in unconsolidated affiliates |  |  | 16.0 |
Targa Resources Partners LP [Member] | Little Missouri 4 [Member] |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |
Return of capital from unconsolidated affiliate |  |  | $ 8.0 |
Targa Resources Partners LP [Member] | T2 LaSalle and T2 Eagle Ford [Member] |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |
Unamortized excess fair value | $ 21.6 |  |  |
Preliminary estimated useful lives of the underlying assets | 20 years |  |  |
GCX DevCo JV [Member] |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |
Ownership interest | 20.00% | 20.00% | 20.00% |
GCX DevCo JV [Member] | GCX [Member] |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |
Ownership interest | 25.00% | 25.00% | 25.00% | 25.00%

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