Fair Value Fair Value of Derivative Contracts (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Cross-currency swap agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net amount	$ 0		$ 0		$ 0
Net amount	(21)		(21)		(52)
Energy commodity derivative contracts(a)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net amount	291		291		590
Net amount	(16)		(16)		(19)
Interest rate swap agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net amount	836		836		377
Net amount	0		0		(17)
Cross-currency swap agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	13		13		0
Contracts available for netting	13		13		0
Balance sheet liability fair value measurements by level	(34)		(34)		(52)
Contracts available for netting	13		13		0
Cross-currency swap agreements | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	0		0		0
Balance sheet liability fair value measurements by level	0		0		0
Cross-currency swap agreements | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	13		13		0
Balance sheet liability fair value measurements by level	(34)		(34)		(52)
Cross-currency swap agreements | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	0		0		0
Balance sheet liability fair value measurements by level	0		0		0
Cross-currency swap agreements | Cash collateral held(b)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash collateral held(b)	0		0		0
Collateral posted(c)	0		0		0
Interest rate swap agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	836		836		385
Contracts available for netting	0		0		8
Balance sheet liability fair value measurements by level	0		0		(25)
Contracts available for netting	0		0		8
Interest rate swap agreements | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	0		0		0
Balance sheet liability fair value measurements by level	0		0		0
Interest rate swap agreements | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	836		836		385
Balance sheet liability fair value measurements by level	0		0		(25)
Interest rate swap agreements | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	0		0		0
Balance sheet liability fair value measurements by level	0		0		0
Interest rate swap agreements | Cash collateral held(b)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash collateral held(b)	0		0		0
Collateral posted(c)	0		0		0
Energy commodity derivative contracts(a)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	323		323		639
Contracts available for netting	32		32		12
Balance sheet liability fair value measurements by level	(66)		(66)		(31)
Contracts available for netting	32		32		12
Fair Value, Assets Liabilites Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Derivatives-net asset (liability)	(2)	$ (49)	(15)	$ (61)
Total gains or (losses) included in earnings	(3)	0	(9)	0
Settlements	5	12	24	24
Derivatives-net asset (liability)	0	(37)	0	(37)
The amount of total gains or (losses) for the period included in earnings attributable to the change in unrealized gains or (losses) relating to assets held at the reporting date	0	$ 1	0	$ 3
Energy commodity derivative contracts(a) | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	6		6		48
Balance sheet liability fair value measurements by level	(19)		(19)		(4)
Energy commodity derivative contracts(a) | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	317		317		589
Balance sheet liability fair value measurements by level	(47)		(47)		(10)
Energy commodity derivative contracts(a) | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance sheet asset fair value measurements by level	0		0		2
Balance sheet liability fair value measurements by level	0		0		(17)
Energy commodity derivative contracts(a) | Cash collateral held(b)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash collateral held(b)	0		0		37
Collateral posted(c)	$ 18		$ 18		$ 0