Investment in Unconsolidated Affiliates Summarized Income Statement of Equity Method Investment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Equity Method Investments [Line Items]
Operating revenues	$ 620.0	$ 646.0	$ 1,658.0	$ 1,852.0
Cost of natural gas and natural gas liquids	268.0	287.0	717.0	856.0
Operating income	139.0	(975.0)	299.0	(778.0)
Net income	$ 110.1	$ (985.1)	$ 230.8	$ (817.3)