Employee Benefits (Components of Net Periodic Benefit (Credit) Cost) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 2	$ 1	$ 3
Interest cost	19	24	39	48
Expected return on plan assets	(31)	(32)	(61)	(63)
Amortization of prior service credit / actuarial (gain) loss	4	5	8	10
Net periodic benefit (credit) cost	(7)	(1)	(13)	(2)
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	1	1	1	2
Amortization of prior service credit / actuarial (gain) loss	(1)	(1)	(1)	(1)
Net periodic benefit (credit) cost	$ 0	$ 0	$ 0	$ 1