Acquisitions (Narrative) (Details) (USD $) In Millions, unless otherwise specified		0 Months Ended	3 Months Ended	1 Months Ended		3 Months Ended		3 Months Ended
	Mar. 31, 2014	Jan. 30, 2014 2014 Acquisition [Member] MMBoe	Mar. 31, 2014 2014 Acquisition [Member]	Aug. 31, 2013 2013 East Texas Properties [Member]	Jun. 30, 2013 2013 East Texas Properties [Member]	Mar. 31, 2014 2013 East Texas Properties [Member]	Aug. 06, 2013 2013 East Texas Properties [Member] MMBoe	Mar. 31, 2014 ETSWDC [Member]	Dec. 31, 2013 ETSWDC [Member]	Aug. 06, 2013 ETSWDC [Member]	Aug. 05, 2013 ETSWDC [Member]
Business Acquisition [Line Items]
Date of acquisition			Jan. 30, 2014			Aug. 06, 2013
Cash paid for acquisition		$ 32.1		$ 107.8
Proved developed reserves		3.9					5.9
Effective date of acquisition						Jun. 01, 2013
Cost associated with acquisition recorded in acquisition and transaction costs					0.4
Ownership percentage	100.00%							59.00%			24.00%
Environmental liability							0.5
Percentage of voting interest acquired									3.00%	32.00%
Gain from remeasurement of previous ownership								$ 1.3