Acquisitions & Divestitures Supplemental Proforma Results of Operations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Business Acquisition [Line Items]
Total Revenues	$ 491.6us-gaap_OilAndGasRevenue	$ 817.5us-gaap_OilAndGasRevenue
Net income	(55.6)us-gaap_NetIncomeLoss	39.7us-gaap_NetIncomeLoss
Earnings Per Share, Basic	$ (0.32)us-gaap_EarningsPerShareBasic	$ 0.22us-gaap_EarningsPerShareBasic
Earnings Per Share, Diluted	$ (0.32)us-gaap_EarningsPerShareDiluted	$ 0.22us-gaap_EarningsPerShareDiluted
Business Acquisition, Pro Forma Revenue		843.6
Business Acquisition, Pro Forma Net Income (Loss)		$ 46.7
Business Acquisition, Pro Forma Earnings Per Share, Basic		$ 0.26
Business Acquisition, Pro Forma Earnings Per Share, Diluted		$ 0.26

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