Employee Benefit Plans (Quarterly Info) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Components of net periodic benefit cost (credit):
Service cost	$ 14	$ 13
Interest cost	11	15
Expected return on plan assets	(16)	(20)
Amortization of net actuarial loss	6	7
Net periodic benefit cost (credit)	15	15
Employer contributions	2
Estimated future employer contributions in current fiscal year	84
Other Postretirement Benefits [Member]
Components of net periodic benefit cost (credit):
Interest cost	2	2
Expected return on plan assets	(3)	(3)
Amortization of prior service cost (credit)	(1)	(3)
Reclassification to regulatory liability	1	1
Net periodic benefit cost (credit)	(1)	(3)
Amortization of prior service cost (credit) from regulatory assets (liabilities)	(1)	$ (2)
Employer contributions	1
Estimated future employer contributions in current fiscal year	$ 5