Employee Benefit Plans (Amounts Recorded in Accumulated Other Comprehensive Income not yet Recognized) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Pension And Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Before Tax [Roll Forward]
Balance, beginning of year	$ 3,814
Amortization for the period	217	$ 213	$ 191
Deferrals for the period	(1,499)	611
Impact of foreign currency changes and other	23	16
Balance, end of period	4,179	3,814
Pension Benefits | Prior Service Cost
Pension And Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Before Tax [Roll Forward]
Balance, beginning of year	(15)	(22)	(25)
Amortization for the period	4	4	3
Deferrals for the period	0	3	0
Impact of foreign currency changes and other	(1)	0	0
Balance, end of period	(12)	(15)	(22)
Pension Benefits | Net Actuarial (Gain) Loss
Pension And Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Before Tax [Roll Forward]
Balance, beginning of year	3,829	3,611	3,481
Amortization for the period	(217)	(213)	(191)
Deferrals for the period	634	430	323
Impact of foreign currency changes and other	(55)	1	(2)
Balance, end of period	4,191	3,829	3,611
Other Postretirement Benefits
Pension And Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Before Tax [Roll Forward]
Balance, beginning of year	449
Amortization for the period	24	17	36
Deferrals for the period	(124)	96
Impact of foreign currency changes and other	2	(1)
Balance, end of period	406	449
Other Postretirement Benefits | Prior Service Cost
Pension And Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Before Tax [Roll Forward]
Balance, beginning of year	41	10	1
Amortization for the period	(4)	(1)	0
Deferrals for the period	27	32	9
Impact of foreign currency changes and other	1	0	0
Balance, end of period	65	41	10
Other Postretirement Benefits | Net Actuarial (Gain) Loss
Pension And Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Before Tax [Roll Forward]
Balance, beginning of year	408	344	557
Amortization for the period	(24)	(17)	(36)
Deferrals for the period	(45)	82	(179)
Impact of foreign currency changes and other	2	(1)	2
Balance, end of period	$ 341	$ 408	$ 344