Proposed Merger with Dominion Resources (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares
Business Combinations [Abstract]
Business Acquisition, Date of Acquisition Agreement	Jan. 31, 2016
Business Acquisition, Share Price | $ / shares	$ 25.00
Maximum Extension to Merger Agreement	9 months
Business Acquisition, Acquirer Termination Fee	$ 154
Business Acquisition, Acquiree Termination Fee	$ 99
Alternative Transaction Period	12 months
BusinessAcquisitionAcquireeReimbursementFee	$ 5