Accumulated Other Comprehensive Income​ (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent [Abstract]
Balance at beginning of period	$ 1,628	$ 1,389	$ 1,447	$ 2,973
Other comprehensive income:
Total Other comprehensive income, net of tax	21	80	583	139
Balance at end of period	1,578	1,488	1,578	1,488
Pension benefits
Other comprehensive income:
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	5	3	573	58
Pension and other postretirement benefits
AOCI Attributable to Parent [Abstract]
Balance at beginning of period	(1,199)	(1,784)	(1,791)	(43)
Other comprehensive income:
Separation-related adjustments (A)	0	0	0	50
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment and Tax	0	0	0	(1,752)
Other Comprehensive Income (Loss), before Reclassifications, before Tax	34	4	178	(34)
Other Comprehensive Income (Loss) before Reclassifications, Tax	(8)	(1)	(41)	8
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	26	3	137	(26)
Amortization of net actuarial loss and prior service cost	25	36	651	153
Tax expense	(6)	(8)	(151)	(35)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	19	28	500	118
Total Other comprehensive income, net of tax	45	31	637	92
Balance at end of period	(1,154)	(1,753)	(1,154)	(1,753)
Foreign currency translation
AOCI Attributable to Parent [Abstract]
Balance at beginning of period	35	(53)	29	338
Other comprehensive income:
Separation-related adjustments (A)	0	0	0	396
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(11)	37	(5)	20
Total amount reclassified from Accumulated other comprehensive loss, net of tax	0	0	0	22
Total Other comprehensive income, net of tax	(11)	37	(5)	42
Balance at end of period	24	(16)	24	(16)
Cash flow hedges
AOCI Attributable to Parent [Abstract]
Balance at beginning of period	(35)	(11)	1	0
Other comprehensive income:
Separation-related adjustments (A)	0	0	0	4
Other Comprehensive Income (Loss), before Reclassifications, before Tax	(66)	2	(160)	4
Other Comprehensive Income (Loss) before Reclassifications, Tax	15	(1)	37	(1)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(51)	1	(123)	3
Amortization of net actuarial loss and prior service cost	49	14	96	3
Tax expense	(11)	(3)	(22)	(1)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	38	11	74	2
Total Other comprehensive income, net of tax	(13)	12	(49)	5
Balance at end of period	(48)	1	(48)	1
Accumulated other comprehensive (loss) income
AOCI Attributable to Parent [Abstract]
Balance at beginning of period	(1,199)	(1,848)	(1,761)	295
Other comprehensive income:
Total Other comprehensive income, net of tax	21	80	583	139
Balance at end of period	$ (1,178)	$ (1,768)	$ (1,178)	$ (1,768)