Basis of Presentation Impairment of Long-Lived Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment	$ 28.3	$ 5.0	$ 28.4	$ 1,188.2
Proved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment				1,167.9
Unproved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment				16.6
Pinedale [Member] | Proved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment				1,164.0
Other Northern [Member] | Proved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment				3.5
Other Southern [Member] | Proved Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment				0.4
Williston & Permian Basins [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Goodwill impairment				$ 3.7