Investments in Unconsolidated Affiliates Summarized Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Operating Revenues	$ 121.4	$ 72.9	$ 251.7	$ 146.3
Operating Expenses	86.8	36.8	162.1	84.7
Net Income	34.6	36.1	89.6	61.5
Jackalope Gas Gathering Services, L.L.C.
Schedule of Equity Method Investments [Line Items]
Amortization	0.1	0.1	0.1	0.1
Stagecoach Gas Services LLC
Schedule of Equity Method Investments [Line Items]
Operating Revenues	43.1	42.8	127.1	56.0
Operating Expenses	20.4	18.4	58.4	24.1
Net Income	22.7	24.4	68.8	31.9
Other Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Operating Revenues	78.3	30.1	124.6	90.3
Operating Expenses	66.4	18.4	103.7	60.6
Net Income	11.9	11.7	20.8	29.6
Tres Palacios Holdings LLC
Schedule of Equity Method Investments [Line Items]
Amortization	0.9		0.3	0.3
Powder River Basin Industrial Complex, LLC
Schedule of Equity Method Investments [Line Items]
Amortization	$ 0.2	$ 0.4	$ 0.5	$ 1.2