Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 1	$ 2	$ 2
Interest cost	19	19	55	59
Expected return on plan assets	(32)	(30)	(95)	(92)
Amortization of actuarial (gain) loss / (prior service credit)	4	4	14	12
Net periodic benefit cost	(8)	(6)	(24)	(19)
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0	1	1	2
Amortization of actuarial (gain) loss / (prior service credit)	0	(1)	(2)	(1)
Net periodic benefit cost	$ 0	$ 0	$ (1)	$ 1