Acquisitions & Divestitures (Narrative) (Details) $ in Millions	3 Months Ended	4 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($) a
Business Acquisition [Line Items]
Payments to Acquire Businesses Net				$ 941.8
Net acres of producing and undeveloped oil and gas properties | a				26,500
Number of vertical producing wells				270
Escrow Deposit				$ 50.0
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	$ 40.1	$ 61.9	$ 69.6
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	2.4	$ 14.0	0.9
Term Loan due 2017 [Member]
Business Acquisition [Line Items]
Borrowings to Fund Acquisition				300.0
Midcontinent Divestitures [Member]
Business Acquisition [Line Items]
Gain (Loss) on Disposition of Oil and Gas and Timber Property	0.4		(26.4)	(199.4)
Proceeds from Sale of Property, Plant, and Equipment				668.2
Southern Region Divestitures [Member]
Business Acquisition [Line Items]
Gain (Loss) on Disposition of Oil and Gas and Timber Property	$ (1.1)		$ (2.9)	53.3
Proceeds from Sale of Property, Plant, and Equipment				96.3
Permian Basin [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross				$ 941.8