Employee Benefit Plans (Quarterly Info) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Components of net periodic benefit cost (credit):
Service cost	$ 12	$ 13	$ 25	$ 27
Interest cost	14	16	29	31
Expected return on plan assets	(21)	(21)	(41)	(42)
Amortization of net actuarial loss	7	7	14	15
Net actuarial loss from settlements	0	1	0	1
Net periodic benefit cost (credit)	12	16	27	32
Employer contributions			49
Estimated future employer contributions in current fiscal year			34
Other Postretirement Benefits [Member]
Components of net periodic benefit cost (credit):
Service cost	1	1	1	1
Interest cost	2	2	4	4
Expected return on plan assets	(3)	(3)	(6)	(6)
Amortization of prior service cost (credit)	(4)	(5)	(7)	(8)
Reclassification to regulatory liability	1	1	2	2
Net periodic benefit cost (credit)	(3)	(4)	(6)	(7)
Amortization of prior service cost (credit) from regulatory assets (liabilities)	$ (2)	$ (3)	(4)	$ (5)
Employer contributions			3
Estimated future employer contributions in current fiscal year			$ 4