Employee Benefit Plans (Net Periodic Benefit Costs and Other Changes Recognized in OCI) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other changes in plan assets and benefit obligations recognized in OCI:
Income (loss) from discontinued operations, net of income tax	$ 0	$ 0	$ 0	$ 0	$ 0	$ (968)	$ 58	$ (76)	$ 0	$ (986)	$ (2,734)
Total recognized in OCI									(263)	(144)	(131)
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs									223	238	272
Interest costs									391	429	423
Settlement and curtailment costs									(1)	4	2
Expected return on plan assets									(533)	(516)	(527)
Amortization of net actuarial (gains) losses									182	195	189
Amortization of prior service costs (credit)									(3)	(1)	0
Total net periodic benefit costs (credit)									259	349	359
Other changes in plan assets and benefit obligations recognized in OCI:
Net actuarial (gains) losses									244	149	238
Prior service costs (credit)									(110)	(1)	(11)
Amortization of net actuarial (gains) losses									(182)	(195)	(189)
Amortization of prior service (costs) credit									3	1	0
Income (loss) from discontinued operations, net of income tax									0	0	(1)
Disposal of subsidiary									0	(30)	0
Total recognized in OCI									(45)	(76)	37
Total recognized in net periodic benefit costs and OCI									214	273	396
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs									6	6	9
Interest costs									55	76	82
Settlement and curtailment costs									0	2	19
Expected return on plan assets									(71)	(72)	(75)
Amortization of net actuarial (gains) losses									(34)	0	10
Amortization of prior service costs (credit)									(20)	(22)	(6)
Total net periodic benefit costs (credit)									(64)	(10)	39
Other changes in plan assets and benefit obligations recognized in OCI:
Net actuarial (gains) losses									(248)	(146)	(124)
Prior service costs (credit)									(7)	0	(41)
Amortization of net actuarial (gains) losses									34	0	(10)
Amortization of prior service (costs) credit									20	22	6
Income (loss) from discontinued operations, net of income tax									0	0	1
Disposal of subsidiary									0	2	0
Total recognized in OCI									(201)	(122)	(168)
Total recognized in net periodic benefit costs and OCI									$ (265)	$ (132)	$ (129)