Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 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	$ 0.2	$ 0.5
Fair Value of Derivative Liabilities	1.6	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	0.9
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	0.2	0.5
Fair Value of Derivative Liabilities	1.6	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	0.9
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	0.6	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	0.6	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	253.8	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	239.6	239.1
Carrying value of senior notes	222.6	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	422.0	409.0
Carrying value of senior notes	$ 404.7	$ 404.9