Employee Benefit Plans - Balance Sheet Position (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Pension and Other Postretirement Defined Benefit Plans, Liabilities [Abstract]
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	$ (15,138)	$ (12,392)
Pension Benefits
Pension and Other Postretirement Defined Benefit Plans, Liabilities [Abstract]
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	(28)	(22)
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	(12,473)	(10,101)
Pension and Other Postretirement Defined Benefit Plans, Liabilities	(12,501)	(10,123)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, before Tax, [Abstract]
Pension and other postretirement benefit plans, accumulated other comprehensive income (loss), net actuarial loss, before tax	(8,409)	(5,349)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	0	0
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	(8,409)	(5,349)
Other Postretirement and Postemployment Benefits
Pension and Other Postretirement Defined Benefit Plans, Liabilities [Abstract]
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	(139)	(139)
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	(2,366)	(2,023)
Pension and Other Postretirement Defined Benefit Plans, Liabilities	(2,505)	(2,162)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, before Tax, [Abstract]
Pension and other postretirement benefit plans, accumulated other comprehensive income (loss), net actuarial loss, before tax	(465)	(103)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	135	161
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	$ (330)	$ 58