Pension Plans and Other Postretirement Benefits - Changes in Level 3 Pension Plan Assets (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets:
Fair value of plan assets, beginning of year	$ 5,663	$ 0
Actual return on plan assets	1,064	(17)
Fair value of plan assets, end of year	6,395	5,663
Level 3
Change in plan assets:
Fair value of plan assets, beginning of year	430
Actual return on plan assets	5
Purchases, sales and settlements	(82)
Transfers into (out of) Level 3	1
Fair value of plan assets, end of year	354	430
Real estate | Level 3
Change in plan assets:
Fair value of plan assets, beginning of year	425
Actual return on plan assets	5
Purchases, sales and settlements	(77)
Transfers into (out of) Level 3	0
Fair value of plan assets, end of year	353	425
U.S. corporate securities | Level 3
Change in plan assets:
Fair value of plan assets, beginning of year	5
Actual return on plan assets	0
Purchases, sales and settlements	(5)
Transfers into (out of) Level 3	1
Fair value of plan assets, end of year	$ 1	$ 5