Employee Benefit Plans (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Qualified Pension Plan
Net periodic cost consists of:
Service cost	$ 28	$ 26
Interest cost	43	45
Expected return on plan assets	(76)	(71)
Amortization of prior service credit		(1)
Amortization of actuarial losses		12
Net periodic cost/(benefit)	(5)	11
Nonqualified Pension Plan
Net periodic cost consists of:
Service cost	1	1
Interest cost	2	3
Amortization of actuarial losses	1	1
Net periodic cost/(benefit)	4	5
Postretirement Benefits
Net periodic cost consists of:
Service cost	1	1
Interest cost	3	4
Expected return on plan assets	(1)	(1)
Net periodic cost/(benefit)	$ 3	$ 4