Pension and Other Postretirement Employee Benefit Plans - Changes In Level 3 Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
U.S. defined benefit pension plans
Change in plan assets
Fair value of plan assets, beginning of year	$ 14,125	$ 14,623
Fair value of plan assets, end of year	14,272	14,125	$ 14,623
U.S. defined benefit pension plans | Equity securities
Change in plan assets
Fair value of plan assets, beginning of year	4,833
Fair value of plan assets, end of year	5,192	4,833
U.S. defined benefit pension plans | Corporate debt securities
Change in plan assets
Fair value of plan assets, beginning of year	1,621
Fair value of plan assets, end of year	1,795	1,621
U.S. defined benefit pension plans | Mortgage-backed securities
Change in plan assets
Fair value of plan assets, beginning of year	68
Fair value of plan assets, end of year	104	68
U.S. defined benefit pension plans | Other
Change in plan assets
Fair value of plan assets, beginning of year	2,321
Fair value of plan assets, end of year	1,664	2,321
OPEB plans
Change in plan assets
Fair value of plan assets, beginning of year	1,855	1,903
Fair value of plan assets, end of year	1,956	1,855	1,903
Level 3 | U.S. defined benefit pension plans
Change in plan assets
Fair value of plan assets, beginning of year	539	351	441
Actual return on plan assets, Realized gains/(losses)	0	0	(2)
Actual return on plan assets, Unrealized gains/(losses)	(157)	195	(91)
Purchases, sales and settlements, net	0	(7)	5
Transfers in and/or out of level 3	14	0	(2)
Fair value of plan assets, end of year	396	539	351
Level 3 | U.S. defined benefit pension plans | Equity securities
Change in plan assets
Fair value of plan assets, beginning of year	2	4	4
Actual return on plan assets, Realized gains/(losses)	0	0	0
Actual return on plan assets, Unrealized gains/(losses)	0	(2)	0
Purchases, sales and settlements, net	0	0	0
Transfers in and/or out of level 3	0	0	0
Fair value of plan assets, end of year	2	2	4
Level 3 | U.S. defined benefit pension plans | Corporate debt securities
Change in plan assets
Fair value of plan assets, beginning of year	2	9	7
Actual return on plan assets, Realized gains/(losses)	0	0	(2)
Actual return on plan assets, Unrealized gains/(losses)	0	0	2
Purchases, sales and settlements, net	1	(7)	4
Transfers in and/or out of level 3	1	0	(2)
Fair value of plan assets, end of year	4	2	9
Level 3 | U.S. defined benefit pension plans | Mortgage-backed securities
Change in plan assets
Fair value of plan assets, beginning of year	1	1	0
Actual return on plan assets, Realized gains/(losses)	0	0	0
Actual return on plan assets, Unrealized gains/(losses)	0	0	0
Purchases, sales and settlements, net	(1)	0	1
Transfers in and/or out of level 3	0	0	0
Fair value of plan assets, end of year	0	1	1
Level 3 | U.S. defined benefit pension plans | Other
Change in plan assets
Fair value of plan assets, beginning of year	534	337	430
Actual return on plan assets, Realized gains/(losses)	0	0	0
Actual return on plan assets, Unrealized gains/(losses)	(157)	197	(93)
Purchases, sales and settlements, net	0	0	0
Transfers in and/or out of level 3	13	0	0
Fair value of plan assets, end of year	390	534	337
Level 3 | OPEB plans
Change in plan assets
Fair value of plan assets, beginning of year	1,855	1,903	1,749
Actual return on plan assets, Realized gains/(losses)	0	0	0
Actual return on plan assets, Unrealized gains/(losses)	102	(48)	154
Purchases, sales and settlements, net	0	0	0
Transfers in and/or out of level 3	0	0	0
Fair value of plan assets, end of year	$ 1,957	$ 1,855	$ 1,903