Investments in Unconsolidated Affiliates - Activity Related to Partnership's Investments in Unconsolidated Affiliates (Parenthetical) (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Schedule Of Equity Method Investments [Line Items]
Return of capital from unconsolidated affiliates	$ 2.2	$ 2.2
Contributions of property, plant and equipment	16.0	$ 1.0
Targa Resources Partners LP [Member] \| Gulf Coast Fractionators LP [Member]
Schedule Of Equity Method Investments [Line Items]
Return of capital from unconsolidated affiliates	2.2
Targa Resources Partners LP [Member] \| Little Missouri 4 [Member]
Schedule Of Equity Method Investments [Line Items]
Return of capital from unconsolidated affiliates	8.0
Targa Resources Partners LP [Member] \| T2 Joint Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Unamortized excess fair value	$ 25.0
Preliminary estimated useful lives of the underlying assets	20 years

Contribution of Property plant and equipment to investment in unconsolidated affiliates.

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- Definition

The estimated periods of time over which underlying assets amortized on a straight-line basis.

+ References

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- Definition

Difference between amount at which an investment accounted for under the equity method of accounting is carried (reported) on the balance sheet and amount of underlying equity in net assets the reporting Entity has in the investee.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

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- Definition

Amount of distribution received from equity method investee for return of investment, classified as investing activities. Excludes distribution for return on investment, classified as operating activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 21D
-URI http://asc.fasb.org/extlink&oid=109223946&loc=SL94080555-108585

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
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-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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