Employee Benefit Plans (Quarterly Info) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 14	$ 10
Interest cost	15	16
Expected return on plan assets	(19)	(19)
Amortization of net actuarial loss	11	9
Net periodic benefit cost (credit)	21	16
Employer contributions	16
Estimated future employer contributions in current fiscal year	47
Other Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	1	1
Interest cost	2	2
Expected return on plan assets	(3)	(3)
Amortization of prior service cost (credit)	(4)	(5)
Reclassification to regulatory liability	1	1
Net periodic benefit cost (credit)	(3)	(4)
Amortization of prior service cost (credit) from regulatory assets (liabilities)	(2)	(3)
Employer contributions	1
Estimated future employer contributions in current fiscal year	$ 6