Investment in Unconsolidated Affiliates Summarized Income Statement of Equity Method Investment (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Equity Method Investments [Line Items]
Equity in earnings of unconsolidated affiliates	$ 29.3	$ 29.4	$ 63.2	$ 65.0
Operating revenues	805.0	626.0	1,553.0	1,292.0
Cost of natural gas and NGLs	444.0	279.0	819.0	587.0
Operating income	126.0	122.0	265.0	262.0
Net income	$ 86.4	$ 86.2	$ 191.0	$ 197.4

Equity Method Investment, Summarized Financial Information, Operating Income

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