Derivative Financial Instruments - Oil and Gas Commodity Contracts (Details) (Swap, USD $) In Millions, unless otherwise specified	Sep. 30, 2013
Derivative [Line Items]
September 30, 2013	$ 1.9
Natural Gas Commodity Contract
Derivative [Line Items]
Natural Gas, Volume	16,018.0
September 30, 2013	3.6
Natural Gas Commodity Contract | Remainder of 2013
Derivative [Line Items]
Natural Gas, Volume	5,141.0
Weighted average strike price per Mmbtu/Bbl	3.72
September 30, 2013	0.6
Natural Gas Commodity Contract | 2014
Derivative [Line Items]
Natural Gas, Volume	10,877.0
Weighted average strike price per Mmbtu/Bbl	4.14
September 30, 2013	3.0
Oil Commodity Contract
Derivative [Line Items]
September 30, 2013	(1.7)
Derivative, Commodity Contract, Oil Volume	375,000.0
Oil Commodity Contract | Remainder of 2013
Derivative [Line Items]
Weighted average strike price per Mmbtu/Bbl	94.05
September 30, 2013	(0.8)
Derivative, Commodity Contract, Oil Volume	103,000.0
Oil Commodity Contract | 2014
Derivative [Line Items]
Weighted average strike price per Mmbtu/Bbl	91.87
September 30, 2013	$ (0.9)
Derivative, Commodity Contract, Oil Volume	272,000.0