Derivative Instruments (Details 2) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative assets, current	$ 1,858	$ 2,058
Long-term derivative assets	711	439
Total derivative assets	2,569	2,497
Derivative liabilities, current	1,643	1,782
Long-term derivative liabilities	572	444
Total derivative liabilities	2,215	2,226
Derivative, Fair Value, Net	354	271
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	4	4
Total derivative liabilities	117	112
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	2,565	2,493
Total derivative liabilities	2,098	2,114
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, current	0	0
Derivative Assets, Noncurrent	4	4
Total derivative assets	4	4
Current derivative liabilities	43	44
Derivative Liabilities, Noncurrent	75	70
Total derivative liabilities	118	114
Derivative, Fair Value, Net	(114)	(110)
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	4	4
Total derivative liabilities	117	112
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	0	0
Total derivative liabilities	1	2
Energy Related Derivative [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, current	1,858	2,058
Derivative Assets, Noncurrent	707	435
Total derivative assets	2,565	2,493
Current derivative liabilities	1,600	1,738
Derivative Liabilities, Noncurrent	497	374
Total derivative liabilities	2,097	2,112
Derivative, Fair Value, Net	468	381
Energy Related Derivative [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets	2,565	2,493
Total derivative liabilities	$ 2,097	$ 2,112