Net realized and unrealized gains and losses on derivative transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Commodity contracts: [Abstract]
Electricity	$ 72	$ 13	$ 78
Natural Gas	103	72	28
Realized And Unrealized Losses Net Commodity Contracts, Foreign Currency	$ 1	$ 0	$ 1