Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Retirement Benefits [Abstract]
Service cost	$ 1	$ 1	$ 2	$ 2
Interest cost	18	19	54	56
Expected return on assets	(31)	(32)	(94)	(95)
Amortization of prior service credit / actuarial loss	4	4	13	12
Net periodic benefit cost	$ (8)	$ (8)	$ (25)	$ (25)