Benefit Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended		3 Months Ended		9 Months Ended
	Jun. 30, 2014	Sep. 30, 2014 Pension Benefits [Member]	Sep. 30, 2013 Pension Benefits [Member]	Sep. 30, 2014 Pension Benefits [Member]	Sep. 30, 2013 Pension Benefits [Member]	Sep. 30, 2014 Other Postretirement Benefits [Member]	Sep. 30, 2013 Other Postretirement Benefits [Member]	Sep. 30, 2014 Other Postretirement Benefits [Member]	Sep. 30, 2013 Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 5	$ 6	$ 13	$ 18		$ 1		$ 1
Interest cost		37	34	111	101	1	1	3	3
Expected return on plan assets		(52)	(49)	(157)	(148)	(1)	(1)	(3)	(3)
Amortization of unrecognized net loss		7	12	22	40		1		1
Amortization of unrecognized prior service benefit						(2)	(6)	(15)	(19)
Regulatory asset decrease			4	1	4
Curtailment gain	(86)							(86)
Net periodic benefit cost		$ (3)	$ 7	$ (10)	$ 15	$ (2)	$ (4)	$ (101)	$ (17)