Benefit Plans - Rollforward of level 3 plan assets (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in pension plan assets
Fair value of plan assets, beginning of year	$ 5,299	$ 6,284
Fair value of plan assets, end of year	6,192	5,299	$ 6,284
Significant unobservable inputs (Level 3)
Change in pension plan assets
Fair value of plan assets, beginning of year	5	39	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	3	0
Purchases, sales and settlements, net	(5)	(5)	29
Net transfers in and/or (out) of Level 3	0	(32)	0
Fair value of plan assets, end of year	0	5	39
Significant unobservable inputs (Level 3) | Equity securities
Change in pension plan assets
Fair value of plan assets, beginning of year	0	29	0
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	3	0
Purchases, sales and settlements, net	0	0	29
Net transfers in and/or (out) of Level 3	0	(32)	0
Fair value of plan assets, end of year	0	0	29
Significant unobservable inputs (Level 3) | Corporate
Change in pension plan assets
Fair value of plan assets, beginning of year	5	10	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	(5)	(5)	0
Net transfers in and/or (out) of Level 3	0	0	0
Fair value of plan assets, end of year	$ 0	$ 5	$ 10