Derivative Financial Instruments Derivative Financial Instruments - Oil and Gas Commodity Contracts (Details) (Swap [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Derivative [Line Items]
Fair value at March 31, 2013	$ (8.1)
Natural Gas Commodity Contract [Member]
Derivative [Line Items]
Natural Gas, Volume	26,200,000
Fair value at March 31, 2013	(7.0)
Natural Gas Commodity Contract [Member] | Remainder of 2013 [Member]
Derivative [Line Items]
Natural Gas, Volume	15,400,000
Weighted average strike price per Mmbtu/Bbl	3.72
Fair value at March 31, 2013	(6.0)
Natural Gas Commodity Contract [Member] | 2014 [Member]
Derivative [Line Items]
Natural Gas, Volume	10,900,000
Weighted average strike price per Mmbtu/Bbl	4.14
Fair value at March 31, 2013	(1.0)
Oil Commodity Contract [Member]
Derivative [Line Items]
Swap, Volume	600,000
Fair value at March 31, 2013	(1.1)
Oil Commodity Contract [Member] | Remainder of 2013 [Member]
Derivative [Line Items]
Swap, Volume	300,000
Weighted average strike price per Mmbtu/Bbl	94.05
Fair value at March 31, 2013	(0.8)
Oil Commodity Contract [Member] | 2014 [Member]
Derivative [Line Items]
Swap, Volume	300,000
Weighted average strike price per Mmbtu/Bbl	91.87
Fair value at March 31, 2013	$ (0.3)