Benefit Plans - Change in pension plan assets (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Change in pension plan assets
Fair value of plan assets, beginning of year	$ 6,284	$ 5,650
Actual return on plan assets	(300)	1,051
Employer contribution	16	131
Benefits paid	(646)	(553)
Translation adjustment and other	(55)	5
Fair value of plan assets, end of year	$ 5,299	$ 6,284