Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Retirement Benefits [Abstract]
Service cost	$ 1	$ 1	$ 1	$ 1
Interest cost	18	19	36	38
Expected return on assets	(31)	(32)	(63)	(63)
Amortization of prior service credit / actuarial loss	4	4	9	7
Net periodic benefit cost	$ (8)	$ (8)	$ (17)	$ (17)