Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Natural Gas [Member] | Fair Value, Inputs, Total [Member] | Natural Gas excluding NGLs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Assets	$ 0.3	$ 0.5
Fair Value of Derivative Liabilities	2.7	0.7
Natural Gas [Member] | Fair Value, Inputs, Total [Member] | Gas Basis excluding NGLs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Liabilities	0.3
Natural Gas [Member] | Fair Value, Inputs, Level 1 [Member] | Natural Gas excluding NGLs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Assets	0	0
Fair Value of Derivative Liabilities	0	0
Natural Gas [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Basis excluding NGLs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Liabilities	0
Natural Gas [Member] | Fair Value, Inputs, Level 2 [Member] | Natural Gas excluding NGLs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Assets	0.3	0.5
Fair Value of Derivative Liabilities	2.7	0.7
Natural Gas [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Basis excluding NGLs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Liabilities	0.3
Natural Gas [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas excluding NGLs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Assets	0	0
Fair Value of Derivative Liabilities	0	0
Natural Gas [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Basis excluding NGLs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Liabilities	0
Oil [Member] | Fair Value, Inputs, Total [Member] | Crude Oil [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Assets		0.3
Fair Value of Derivative Liabilities	0.8	0.2
Oil [Member] | Fair Value, Inputs, Level 1 [Member] | Crude Oil [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Assets		0
Fair Value of Derivative Liabilities	0	0
Oil [Member] | Fair Value, Inputs, Level 2 [Member] | Crude Oil [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Assets		0.3
Fair Value of Derivative Liabilities	0.8	0.2
Oil [Member] | Fair Value, Inputs, Level 3 [Member] | Crude Oil [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Derivative Assets		0
Fair Value of Derivative Liabilities	0	0
Senior notes due 2017 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of senior notes	253.4	256.7
Carrying value of senior notes	250.0	250.0
Senior notes due 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of senior notes	236.3	239.1
Carrying value of senior notes	222.5	222.4
Senior notes due 2022 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of senior notes	408.2	409.0
Carrying value of senior notes	$ 404.8	$ 404.9