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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Periodic Benefit Cost Amortized From Accumulated Other Comprehensive Income (Loss) [Abstract]
Income (loss) from discontinued operations, net of income tax	$ 0	$ (968)	$ 58	$ (76)	$ (1,370)	$ (435)	$ (1,361)	$ 432	$ (986)	$ (2,734)	$ 1,324
Total recognized in OCI									(144)	(131)	(354)
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs									238	272	275
Interest costs									429	423	414
Settlement and curtailment costs									4	2	(1)
Expected return on plan assets									(516)	(527)	(534)
Amortization of net actuarial (gains) losses									195	189	189
Amortization of prior service costs (credit)									(1)	0	(1)
Total net periodic benefit costs (credit)									349	359	342
Net Periodic Benefit Cost Amortized From Accumulated Other Comprehensive Income (Loss) [Abstract]
Net actuarial (gains) losses									149	238	43
Prior service costs (credit)									(1)	(11)	0
Amortization of net actuarial (gains) losses									(195)	(189)	(189)
Amortization of prior service (costs) credit									1	0	1
Income (loss) from discontinued operations, net of income tax									0	(1)	(2)
Disposal of subsidiary									(30)	0	0
Total recognized in OCI									(76)	37	(147)
Total recognized in net periodic benefit costs and OCI									273	396	195
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs									6	9	17
Interest costs									76	82	89
Settlement and curtailment costs									2	19	3
Expected return on plan assets									(72)	(75)	(80)
Amortization of net actuarial (gains) losses									0	10	42
Amortization of prior service costs (credit)									(22)	(6)	(5)
Total net periodic benefit costs (credit)									(10)	39	66
Net Periodic Benefit Cost Amortized From Accumulated Other Comprehensive Income (Loss) [Abstract]
Net actuarial (gains) losses									(146)	(124)	(161)
Prior service costs (credit)									0	(41)	(7)
Amortization of net actuarial (gains) losses									0	(10)	(42)
Amortization of prior service (costs) credit									22	6	5
Income (loss) from discontinued operations, net of income tax									0	1	(2)
Disposal of subsidiary									2	0	0
Total recognized in OCI									(122)	(168)	(207)
Total recognized in net periodic benefit costs and OCI									$ (132)	$ (129)	$ (141)