Basis of Presentation Impairment of Long-Lived Assets (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment	$ 0.0	$ 0.8	$ 0.1	$ 1,183.2
Goodwill, Acquired During Period			$ 5.3	3.7
Proved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Oil and Gas Properties				1,167.9
Unproved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Oil and Gas Properties				11.6
Pinedale [Member] | Proved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Oil and Gas Properties				1,164.0
Other Northern [Member] | Proved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Oil and Gas Properties				3.5
Other Southern [Member] | Proved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Oil and Gas Properties				0.4
Williston & Permian Basins [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Goodwill, Acquired During Period				$ 3.7