Employee Benefits - Amounts Recognized in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
AOCI Attributable To Parent, Tax [Roll Forward]
Net Change in AOCL	$ (222)	$ (17)	$ 62
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	11,162	13,470
Recognition of Net Periodic Benefits Expense	500	208
Deferrals	155	(341)
Net Change in AOCL	655	(133)	(219)
Balance at end of period	11,840	11,162	13,470
Defined benefit plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,469)	(2,413)
Recognition of Net Periodic Benefits Expense	437	162
Deferrals	291	(218)
Net Change in AOCL	728	(56)
Balance at end of period	(1,741)	(2,469)	(2,413)
Combined Pension Plan | Defined benefit plan
AOCI Attributable To Parent, Tax [Roll Forward]
Balance at beginning of period	755	770
Recognition of Net Periodic Benefits Expense	(137)	47
Deferrals	(59)	32
Net Change in AOCL	(196)	(15)
Balance at end of period	559	755	770
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(2,197)	(2,229)
Recognition of Net Periodic Benefits Expense	423	147
Deferrals	197	(115)
Net Change in AOCL	620	32
Balance at end of period	(1,577)	(2,197)	(2,229)
Combined Pension Plan | Net actuarial loss
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at beginning of period	(2,993)	(3,046)
Recognition of Net Periodic Benefits Expense	186	203
Deferrals	243	(150)
Net Change in AOCL	429	53
Balance at end of period	(2,564)	(2,993)	(3,046)
Combined Pension Plan | Settlement charge
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at beginning of period	0
Recognition of Net Periodic Benefits Expense	383
Deferrals	0
Net Change in AOCL	383
Balance at end of period	383	0
Combined Pension Plan | Prior service cost
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at beginning of period	41	47
Recognition of Net Periodic Benefits Expense	(9)	(9)
Deferrals	13	3
Net Change in AOCL	4	(6)
Balance at end of period	45	41	47
Post-Retirement Benefit Plans | Defined benefit plan
AOCI Attributable To Parent, Tax [Roll Forward]
Balance at beginning of period	90	62
Recognition of Net Periodic Benefits Expense	(5)	5
Deferrals	(31)	33
Net Change in AOCL	(36)	28
Balance at end of period	54	90	62
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(272)	(184)
Recognition of Net Periodic Benefits Expense	14	15
Deferrals	94	(103)
Net Change in AOCL	108	(88)
Balance at end of period	(164)	(272)	(184)
Post-Retirement Benefit Plans | Net actuarial loss
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at beginning of period	(346)	(175)
Recognition of Net Periodic Benefits Expense	4	0
Deferrals	125	(171)
Net Change in AOCL	129	(171)
Balance at end of period	(217)	(346)	(175)
Post-Retirement Benefit Plans | Prior service cost
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at beginning of period	(20)	(71)
Recognition of Net Periodic Benefits Expense	15	16
Deferrals	0	35
Net Change in AOCL	15	51
Balance at end of period	(5)	(20)	(71)
Post-Retirement Benefit Plans | Curtailment loss
AOCI Attributable To Parent, Before Tax [Roll Forward]
Balance at beginning of period	(4)	0
Recognition of Net Periodic Benefits Expense	0	4
Deferrals	0	0
Net Change in AOCL	0	(4)
Balance at end of period	$ (4)	$ (4)	$ 0