Supplemental Oil and Gas Disclosures - Capitalized Costs Related to Oil and Natural Gas (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net capitalized cost:
Proved oil and natural gas properties and equipment		$ 7,932.5	$ 7,882.3	$ 7,924.2
Unproved oil and natural gas properties and equipment			20.2	121.5
Accumulated depreciation, depletion and amortization related to oil, NGLs and natural gas activities	[1]	(7,387.8)	(6,916.2)	(5,557.6)
Net capitalized costs related to producing activities		$ 544.7	$ 986.3	$ 2,488.1

[1] Includes ceiling test write-down in 2016 and 2015.