Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Retirement Benefits [Abstract]
Service cost	$ 1	$ 1
Interest cost	19	18
Expected return on assets	(31)	(32)
Amortization of prior service credit / actuarial loss	2	4
Net benefit cost	(9)	(9)
Settlement charge	113	0
Net benefit cost	$ 104	$ (9)