Wexpro Acquisition of Producing Properties (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 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]
Net cash paid, after post-closing adjustments	$ 104.3
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Proved properties	106.4
Asset retirement obligations	(2.1)
Total fair value of identifiable net assets acquired	104.3
Business Acquisition, Pro Forma Information [Abstract]
Revenue of Acquiree since Acquisition Date, Actual	6.0
Earnings or Loss of Acquiree since Acquisition Date, Actual	1.3
Business Combination, Separately recognized Transactions, Additional Disclosures [Abstract]
Acquisition Cost Expensed	$ 0.3
Acquisition Cost Expensed, Financial Statement Caption	general and administrative expense