Pension and Other Postretirement Benefits - Change in Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Pension Benefits
Change in plan assets:
Fair value of plan assets, beginning of year	$ 2,363	$ 3,191
Actual return (loss) on plan assets	243	(592)
Employer contributions	195	73
Benefits paid	(294)	(309)
Participants’ contributions	0	0
Retiree Medicare drug subsidy	0	0
Fair value of plan assets, end of year	2,507	2,363
Postretirement Benefits
Change in plan assets:
Fair value of plan assets, beginning of year	0	0
Actual return (loss) on plan assets	0	0
Employer contributions	31	34
Benefits paid	(40)	(43)
Participants’ contributions	6	6
Retiree Medicare drug subsidy	3	3
Fair value of plan assets, end of year	$ 0	$ 0