Employee Benefit Plans (Quarterly Info) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 13	$ 14
Interest cost	15	15
Expected return on plan assets	(20)	(21)
Amortization of net actuarial loss	7	8
Net periodic benefit cost (credit)	15	16
Employer contributions	1
Estimated future employer contributions in current fiscal year	62
Other Postretirement Benefits [Member]
Components of net periodic benefit cost:
Interest cost	2	2
Expected return on plan assets	(3)	(3)
Amortization of prior service cost (credit)	(3)	(3)
Reclassification to regulatory liability	1	1
Net periodic benefit cost (credit)	(3)	(3)
Amortization of prior service cost (credit) from regulatory assets (liabilities)	2	$ 2
Employer contributions	1
Estimated future employer contributions in current fiscal year	$ 6