Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 1	$ 1	$ 1
Interest cost	19	19	37	39
Expected return on plan assets	(32)	(31)	(63)	(61)
Amortization of actuarial loss	4	4	9	8
Net periodic benefit cost	(8)	(7)	(16)	(13)
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	0	1	1	1
Amortization of actuarial loss	(1)	(1)	(2)	(1)
Net periodic benefit cost	$ (1)	$ 0	$ (1)	$ 0