Investments in Unconsolidated Affiliates (Net Investments In and Earnings (Loss) from Unconsolidated Affiliates) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Investment	$ 168.0		$ 168.0		$ 943.7
Earnings (loss) from unconsolidated affiliates, net	4.9	$ 10.5	(125.9)	$ 24.4
Stagecoach Gas Services LLC
Schedule of Equity Method Investments [Line Items]
Investment	15.2		15.2		792.5
Earnings (loss) from unconsolidated affiliates, net	0.9	9.9	(139.4)	28.3
Difference between carrying amount and underlying equity	51.3		51.3
Stagecoach Gas Services LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from unconsolidated affiliates, net			155.4
Tres Palacios Holdings LLC
Schedule of Equity Method Investments [Line Items]
Investment	38.4		38.4		35.5
Earnings (loss) from unconsolidated affiliates, net	(0.1)	0.1	9.1	0.2
Difference between carrying amount and underlying equity	21.8		21.8
Amortization	0.3	0.3	0.9	0.9
Powder River Basin Industrial Complex, LLC
Schedule of Equity Method Investments [Line Items]
Investment	3.5		3.5		3.6
Earnings (loss) from unconsolidated affiliates, net	(0.1)	0.0	0.0	(4.4)
Difference between carrying amount and underlying equity		5.5		5.5
Powder River Basin Industrial Complex, LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from unconsolidated affiliates, net				4.5
Crestwood Permian Basin Holdings LLC
Schedule of Equity Method Investments [Line Items]
Investment	110.9		110.9		$ 112.1
Earnings (loss) from unconsolidated affiliates, net	4.2	$ 0.5	4.4	$ 0.3
Difference between carrying amount and underlying equity	$ 7.5		$ 7.5