Basis of Presentation Impairment of Long-Lived Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment	$ 5.0	$ 15.0	$ 1,188.2	$ 35.5
Goodwill, Acquired During Period			3.7
Proved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Oil and Gas Properties			1,167.9	$ 33.8
Unproved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Oil and Gas Properties			16.6
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