Employee Pension and Other Postretirement Benefits - AOCI Loss for Pension (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	$ 1,085	$ 1,027
Prior service costs (credits), Gross	(179)	(121)
Pension and other benefits adjustment, Gross	906	906
Net actuarial loss in AOCI, Tax	427	404
Prior service costs (credits) in AOCI, Tax	(70)	(47)
Pension and other benefits adjustment, Tax	357	357
Net Actuarial (Gain)/Loss, Net of Tax	658	623
Prior service costs (credits), Net of Tax	(109)	(74)
Amounts to be reclassified from accumulated other comprehensive loss	549	549
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	78	82
Prior service costs (credits), Gross	(50)	(15)
Pension and other benefits adjustment, Gross	28	67
Net actuarial loss in AOCI, Tax	32	33
Prior service costs (credits) in AOCI, Tax	(20)	(6)
Pension and other benefits adjustment, Tax	12	27
Net Actuarial (Gain)/Loss, Net of Tax	46	49
Prior service costs (credits), Net of Tax	(30)	(9)
Amounts to be reclassified from accumulated other comprehensive loss	16	40
Executive Supplemental Benefits Pension Plan
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	40	39
Prior service costs (credits), Gross	0	(2)
Pension and other benefits adjustment, Gross	40	37
Net actuarial loss in AOCI, Tax	15	15
Prior service costs (credits) in AOCI, Tax	0	(1)
Pension and other benefits adjustment, Tax	15	14
Net Actuarial (Gain)/Loss, Net of Tax	25	24
Prior service costs (credits), Net of Tax	0	(1)
Amounts to be reclassified from accumulated other comprehensive loss	25	23
Pension and Executive Supplemental Benefits
Pension Plan and Other Postretirement Benefits Plan
Pension and other benefits adjustment, Gross	946	943
Pension and other benefits adjustment, Tax	372	371
Amounts to be reclassified from accumulated other comprehensive loss	$ 574	$ 572