Wexpro Acquisitions of Producing Properties (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Canyon Creek [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Dec. 17, 2014
Business Acquisition, Description of Acquired Entity	an additional interest in natural gas-producing properties in existing Wexpro-operated assets in the Canyon Creek Unit of southwestern Wyoming's Vermillion Basin
Business Combination, Consideration Transferred [Abstract]
Net cash paid	$ 52.4
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Proved properties	54.1
Asset retirement obligations	(1.7)
Total fair value of identifiable net assets acquired	52.4
Business Combination, Separately recognized Transactions, Additional Disclosures [Abstract]
Acquisition Cost Expensed	0.3
Acquisition Cost Expensed, Financial Statement Caption	general and administrative expense
Trail [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Sep. 04, 2013
Business Acquisition, Date of Acquisition Agreement		Jul. 25, 2013
Business Combination, Consideration Transferred [Abstract]
Net cash paid		$ 104.3