Purchase Agreements: (Details) (Precedent Agreement, Transcontinental Gas Pipeline Company, LLC (Transco), USD $)	0 Months Ended
	Apr. 11, 2014	Apr. 11, 2014
Precedent Agreement | Transcontinental Gas Pipeline Company, LLC (Transco)
Purchase Agreements [Line Items]
Base term of an agreement	25 years
Aggregate agreement fixed charge	$ 942,500,000	$ 942,500,000