Acquisitions and Divestitures (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) a	Dec. 31, 2013 USD ($)
Significant Acquisitions and Disposals [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment	$ 21.8	$ 3,296.6	$ 211.1
Payments to Acquire Oil and Gas Property	98.3	960.5	40.9
Permian Basin Acquisition [Member]
Significant Acquisitions and Disposals [Line Items]
Aggregate purchase price		$ 941.8
Net acres of producing and undeveloped oil and gas properties | a		26,500
Vertical producing oil and gas wells		270
Revenue of acquired properties since acquisition date	149.9	$ 159.5
Net income of acquired properties since acquisition date	(2.8)	(438.3)
Permian Basin [Member]
Significant Acquisitions and Disposals [Line Items]
Impairment of Oil and Gas Properties	0.7	467.7
Goodwill [Member]
Significant Acquisitions and Disposals [Line Items]
Impairment of Oil and Gas Properties	14.3		59.5
Nonrecurring Adjustment [Domain]
Significant Acquisitions and Disposals [Line Items]
Gain (Loss) on Sale of Properties, before Applicable Income Taxes	(9.3)
Non-core properties Divestitures [Member]
Significant Acquisitions and Disposals [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment	31.7	783.8	205.8
Gain (Loss) on Sale of Properties, before Applicable Income Taxes	$ 21.0	$ (147.0)	$ 105.7