Pension Benefits (Companies' Net Periodic Benefit Costs) (Detail) (Pension Benefits [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - including administrative expenses	$ 59	$ 47	$ 118	$ 94
Interest cost on projected benefit obligation	137	140	274	280
Expected return on plan assets	(177)	(183)	(352)	(366)
Amortization of net actuarial loss	177	132	354	264
Amortization of prior service costs	2	2	4	4
NET PERIODIC BENEFIT COST	198	138	398	276
Amortization of regulatory asset	1	1	1	1
TOTAL PERIODIC BENEFIT COST	199	139	399	277
Cost capitalized	(68)	(48)	(135)	(96)
Reconciliation to rate level	3	(6)	(32)	(57)
Cost charged to operating expenses	134	85	232	124
Estimated future employer contributions	775		775
Funds made to non-qualified supplemental plan			$ 12