Income Taxes - Summary of Effective Tax Rates (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Tax provision (benefit) at the U.S. federal statutory income tax rate									$ 363		$ (1,334)		$ (1,315)
State and local taxes, net of Federal benefit									9		(19)		(19)
Adjustment with respect to tax for prior periods									11	[1]	(51)	[1]	5	[1]
Adjustment of tax rate used to value deferred taxes									(5)		(7)		0
Change in valuation allowance and reserves									(11)	[2]	15	[2]	(130)	[2]
Uncertain tax adjustments									(10)	[3]	(15)	[3]	0	[3]
Other non-deductible/non-taxable items									(29)	[4]	0	[4]	29	[4]
Other									(3)		5		(1)
Total income provision (benefit)	$ (51)	$ 91	$ 133	$ 152	$ (81)	$ (98)	$ (939)	$ (288)	$ 325		$ (1,406)		$ (1,431)
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Tax benefit at the U.S. federal statutory income tax rate, percent									35.00%		(35.00%)		(35.00%)
State and local taxes, net of Federal benefit, percent									0.90%		(0.50%)		(0.50%)
Adjustment with respect to tax for prior periods, percent									1.10%	[1]	(1.30%)	[1]	0.10%	[1]
Adjustment of tax rate used to value deferred taxes, percent									(0.50%)		(0.20%)		0.00%
Change in valuation allowance reserves, percent									(1.10%)	[2]	0.40%	[2]	(3.50%)	[2]
Uncertain tax adjustments, percent									(1.00%)	[3]	(0.40%)	[3]	0.00%	[3]
Other non-deductible/non-taxable items, percent									(2.80%)	[4]	0.00%	[4]	0.80%	[4]
Other, percent									(0.30%)		0.10%		0.00%
Total income tax benefit, percent									31.30%		(36.90%)		(38.10%)

[1] For 2013, 2012 and 2011, the amount relates to corrections to current and deferred tax balance sheet accounts and changes in estimates as a result of filing the Federal and state income tax returns.

[2] For 2013 and 2012, the amounts relate to changes in valuation allowance in states with net operating loss carryforward periods of 12 to 20 years. For 2011, the amount relates mainly to the release of a valuation allowance previously established on foreign tax credits.

[3] For 2013 and 2012, the amounts primarily relate to the conclusion of state audits and expiration of state statutes of limitations.

[4] For 2013, the amount includes a change in the estimated deductibility of accrued costs for certain regulatory matters that were accrued during 2011.