Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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Interest expense on all other items not previously classified. For example, includes dividends associated with redeemable preferred stock of a subsidiary that is treated as a liability in the parent's consolidated balance sheet.

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Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

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Basis of Presentation of Interim Consolidated Financial Statements (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Asset Impairment Charges	$ 15.0	$ 0.1	$ 35.5	$ 3.6
Unrecognized Tax Benefits	15.6		15.6
Interest expense related to uncertain tax position			0.3
Proved Property [Member]
Asset Impairment Charges	14.4		33.8
Unproved Property [Member]
Asset Impairment Charges	0.6		1.7
Other Northern [Member]
Asset Impairment Charges	13.1		18.0
Midcontinent [Member]
Asset Impairment Charges	1.0		15.5
Permian Basin [Member]
Asset Impairment Charges	$ 0.3		$ 0.3