Employee Benefit Plans (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Qualified Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 26	$ 26	$ 77	$ 77
Interest cost	45	46	134	139
Expected return on plan assets	(71)	(70)	(213)	(211)
Amortization of prior service cost/(credit)	(1)	(2)	(3)	(5)
Amortization of actuarial losses	11	12	33	34
Net periodic cost/(benefit)	10	12	28	34
Nonqualified Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	2	2
Interest cost	2	3	8	9
Amortization of actuarial losses	1	1	3	3
Net periodic cost/(benefit)	4	5	13	14
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	1	4	4
Interest cost	3	4	10	11
Expected return on plan assets	(2)	(1)	(4)	(3)
Amortization of prior service cost/(credit)			(1)	(1)
Net periodic cost/(benefit)	$ 3	$ 4	$ 9	$ 11