Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Offsetting Assets [Line Items]
Derivative assets	$ 496	$ 549
Gross amounts offset in the Consolidated Statements of Financial Position, Assets	(355)	(468)
Derivative assets, net	141	81
Derivative liabilities	(491)	(656)
Gross amounts offset in the Consolidated Statements of Financial Position, Liabilities	365	489
Derivative liabilities, net	(126)	(167)
Natural Gas
Offsetting Assets [Line Items]
Derivative assets	304	348
Gross amounts offset in the Consolidated Statements of Financial Position, Assets	(206)	(306)
Derivative assets, net	98	42
Derivative liabilities	(289)	(461)
Gross amounts offset in the Consolidated Statements of Financial Position, Liabilities	202	321
Derivative liabilities, net	(87)	(140)
Electricity
Offsetting Assets [Line Items]
Derivative assets	186	193
Gross amounts offset in the Consolidated Statements of Financial Position, Assets	(146)	(157)
Derivative assets, net	40	36
Derivative liabilities	(197)	(189)
Gross amounts offset in the Consolidated Statements of Financial Position, Liabilities	158	163
Derivative liabilities, net	(39)	(26)
Other
Offsetting Assets [Line Items]
Derivative assets	1	2
Gross amounts offset in the Consolidated Statements of Financial Position, Assets	0	0
Derivative assets, net	1	2
Derivative liabilities	(3)	(3)
Gross amounts offset in the Consolidated Statements of Financial Position, Liabilities	3	2
Derivative liabilities, net	0	(1)
Foreign currency exchange contracts
Offsetting Assets [Line Items]
Derivative assets	5	6
Gross amounts offset in the Consolidated Statements of Financial Position, Assets	(3)	(5)
Derivative assets, net	2	1
Derivative liabilities	(2)	(3)
Gross amounts offset in the Consolidated Statements of Financial Position, Liabilities	2	3
Derivative liabilities, net	$ 0	$ 0