Accumulated Other Comprehensive Loss (Summary of Changes in AOCI) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (9,519)	$ (6,937)	$ (9,333)	$ (7,525)
Amounts reclassified, pretax - Foreign Currency Translation	1	0
Tax (benefit) expense reclassified - Foreign Currency Translation	(13)	130
Other comprehensive (loss) income before reclassifications, net - Pension	(1)	8
Amounts reclassified, pretax - Pension	44	88
Tax (benefit) expense reclassified - Pension	(10)	(23)
Other comprehensive (loss) income before reclassifications, net - AFS Securities	0	0
Amounts reclassified, pretax - AFS Securities	0	0
Tax (benefit) expense reclassified - AFS Securities	0	0
Other Comprehensive Income (Loss), ASU 2016-01 adoption impact on Securities Arising During Period	0	(5)
Other comprehensive (loss) income before reclassifications, net - Unrealized Hedging (Losses) Gains	7	45
Amounts reclassified, pretax - Unrealized Hedging (Losses) Gains	4	(27)
Tax (benefit) expense reclassified - Unrealized Hedging (Losses) Gains	(3)	(4)
Other comprehensive (loss) income before reclassifications, net	536	429
Amounts reclassified, pretax	49	61
Tax (benefit) expense reclassified	(26)	103
Other Comprehensive Income (Loss), ASU 2016-01 adoption impact on foreign currency translation Arising During Period		0
Other Comprehensive Income (Loss), ASU 2016-01 adoption impact on Defined Benefit Plan Arising During Period		0
Other Comprehensive Income (Loss), ASU 2016-01 adoption impact on Derivatives Arising During Period		0
Reclassification from OCI, current period, ASU 2016-01 adoption impact		(5)
Other Comprehensive Income (Loss), ASU 2018-02 adoption impact on Securities Arising During Period	0
Other Comprehensive Income (Loss), ASU 2018-02 adoption impact on foreign currency translation Arising During Period	(8)
Other Comprehensive Income (Loss), ASU 2018-02 adoption impact on Defined Benefit Plan Arising During Period	(737)
Other Comprehensive Income (Loss), ASU 2018-02 adoption impact on Derivatives Arising During Period	0
Reclassification from OCI, current period, ASU 2018-02 adoption impact	(745)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(2,932)	(2,444)	(3,442)	(2,950)
Other comprehensive (loss) income before reclassifications, net - Foreign Currency Translation	530	376
Defined Benefit Pension and Post-retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(6,422)	(4,579)	(5,718)	(4,652)
Unrealized Gains (Losses) on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	0	0	0	5
Unrealized hedging (losses) gains
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (165)	$ 86	$ (173)	$ 72