Wexpro Acquisition of Producing Properties (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Sep. 04, 2013
Business Acquisition, Date of Acquisition Agreement	Jul. 25, 2013
Business Acquisition, Description of Acquired Entity	an additional interest in natural gas-producing properties in the Trail Unit of southwestern Wyoming's Vermillion Basin
Business Combination, Consideration Transferred [Abstract]
Cash paid at closing	$ 106.7
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Proved properties	108.8	108.8
Asset retirement obligations	(2.1)	(2.1)
Total fair value of identifiable net assets acquired	106.7	106.7
Business Acquisition, Income Information [Abstract]
Revenue of Acquiree since Acquisition Date, Actual	1.5
Earnings or Loss of Acquiree since Acquisition Date, Actual	0.3
Business Combination, Separately recognized Transactions, Additional Disclosures [Abstract]
Acquisition Cost Expensed		$ 0.2
Acquisition Cost Expensed, Financial Statement Caption		general and administrative expense