Employee Pension and Other Postretirement Benefits - Changes in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 18,355	$ 17,386	$ 16,593
Ending balance	16,580	18,355	17,386
Recognized net actuarial gain (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications	71	55	54
Amortization of prior service credit
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications	(30)	(29)	(18)
Pension Benefits | Recognized net actuarial gain (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	905	1,085	1,027
Other comprehensive income before reclassifications	218	(101)	133
Amounts reclassified from AOCI	87	79	75
Ending balance	1,036	905	1,085
Pension Benefits | Amortization of prior service credit
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(152)	(179)	(121)
Other comprehensive income before reclassifications	0	0	(76)
Amounts reclassified from AOCI	(26)	(27)	(18)
Ending balance	(126)	(152)	(179)
Other Benefits | Recognized net actuarial gain (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	51	78	82
Other comprehensive income before reclassifications	18	(19)	6
Amounts reclassified from AOCI	5	8	10
Ending balance	64	51	78
Other Benefits | Amortization of prior service credit
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(29)	(50)	(15)
Other comprehensive income before reclassifications	0	0	(47)
Amounts reclassified from AOCI	(15)	(21)	(12)
Ending balance	$ (14)	$ (29)	$ (50)