Employee Pension and Other Postretirement Benefits - AOCI Loss for Pension (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plan and Other Postretirement Benefits Plan
Amortization period of net actuarial losses	9 years
Excess of market value of plan assets	$ 85
Assetsmoothingperiodofnonamortizing amount	4 years
Amortization period of prior service credit	8 years 5 months 16 days
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	$ 1,027	$ 1,044	$ 467	$ 1,042
Prior service costs (credits), Gross	(121)	(138)
Defined Benefit Plan, Future Amortization of Gain (Loss)	(109)
Pension and other benefits adjustment, Gross	906	906
Net actuarial loss in AOCI, Tax	404	411
Prior service costs (credits) in AOCI, Tax	(47)	(54)
Pension and other benefits adjustment, Tax	357	357
Net Actuarial (Gain)/Loss, Net of Tax	623	633
Prior service costs (credits), Net of Tax	(74)	(84)
Amounts to be reclassified from accumulated other comprehensive loss	549	549
Expected cash contribution to Pension Plan in 2014	150
Not subject to amortization [Member]
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	392
Subject to amortization [Member]
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	635
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	82	62	$ 17	$ 80
Prior service costs (credits), Gross	(15)	(23)
Pension and other benefits adjustment, Gross	67	39
Net actuarial loss in AOCI, Tax	33	25
Prior service costs (credits) in AOCI, Tax	(6)	(9)
Pension and other benefits adjustment, Tax	27	16
Net Actuarial (Gain)/Loss, Net of Tax	49	37
Prior service costs (credits), Net of Tax	(9)	(14)
Amounts to be reclassified from accumulated other comprehensive loss	40	23
Executive Supplemental Benefits Pension Plan
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross	39	39
Prior service costs (credits), Gross	(2)	(2)
Pension and other benefits adjustment, Gross	37	37
Net actuarial loss in AOCI, Tax	15	15
Prior service costs (credits) in AOCI, Tax	(1)	(1)
Pension and other benefits adjustment, Tax	14	14
Net Actuarial (Gain)/Loss, Net of Tax	24	24
Prior service costs (credits), Net of Tax	(1)	(1)
Amounts to be reclassified from accumulated other comprehensive loss	23	23
Pension and Executive Supplemental Benefits
Pension Plan and Other Postretirement Benefits Plan
Pension and other benefits adjustment, Gross	943	943
Pension and other benefits adjustment, Tax	371	371
Amounts to be reclassified from accumulated other comprehensive loss	572	$ 572
Forecast | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan, Future Amortization of Gain (Loss)	$ (76)