Benefit Plans - Rollforward of level 3 plan assets (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets
Fair value of plan assets, beginning of year	$ 6,192	$ 5,299
Fair value of plan assets, end of year	6,987	6,192	$ 5,299
Significant unobservable inputs (Level 3)
Change in plan assets
Fair value of plan assets, beginning of year	0	5	39
Actual return on plan assets: Relating to assets sold during the period	0	0	0
Actual return on plan assets: Relating to assets still held at the reporting date	0	0	3
Purchases, sales and settlements, net	2	(5)	(5)
Net transfers in (out) of Level 3	0	0	(32)
Fair value of plan assets, end of year	2	0	5
Significant unobservable inputs (Level 3) | Equity securities
Change in plan assets
Fair value of plan assets, beginning of year		0	29
Actual return on plan assets: Relating to assets sold during the period			0
Actual return on plan assets: Relating to assets still held at the reporting date			3
Purchases, sales and settlements, net			0
Net transfers in (out) of Level 3			(32)
Fair value of plan assets, end of year			0
Significant unobservable inputs (Level 3) | Corporate
Change in plan assets
Fair value of plan assets, beginning of year	0	5	10
Actual return on plan assets: Relating to assets sold during the period	0	0	0
Actual return on plan assets: Relating to assets still held at the reporting date	0	0	0
Purchases, sales and settlements, net	2	(5)	(5)
Net transfers in (out) of Level 3	0	0	0
Fair value of plan assets, end of year	$ 2	$ 0	$ 5