Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Retirement Benefits [Abstract]
Service cost	$ 1	$ 1	$ 2	$ 2
Interest cost	16	18	49	54
Expected return on assets	(27)	(31)	(81)	(94)
Amortization of prior service credit / actuarial loss	3	4	7	13
Net periodic benefit cost	(7)	(8)	(23)	(25)
Settlement charge	0	0	113	0
Net benefit cost	$ (7)	$ (8)	$ 90	$ (25)