Income Taxes - Summary of Effective Tax Rates (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Tax benefit at the U.S. federal statutory income tax rate	$ 160		$ (278)
State and local taxes, net of Federal benefit	4		(4)
Validation of tax accounts	(10)	[1]	0	[1]
Adjustment of tax rate used to value deferred taxes	(5)		(7)
Change in valuation allowance reserves	10	[2]	8	[2]
Accrual of tax reserves	(5)	[3]	(4)	[3]
Other	(2)		(3)
Total income tax expense (benefit)	$ 152		$ (288)
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%)
State and local taxes, net of Federal benefit, percent	0.90%		(0.50%)
Validation of tax accounts, percent	(2.20%)	[1]	0.00%	[1]
Adjustment of tax rate used to value deferred taxes	(1.10%)		(0.90%)
Change in valuation allowance reserves, percent	2.20%	[2]	0.90%	[2]
Accrual of tax reserves(3)	(1.10%)	[3]	(0.50%)	[3]
Other, percent	(0.40%)		(0.30%)
Total income tax benefit, percent	33.30%		(36.30%)
Minimum [Member]
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Net Operating Loss Carryforward Period	15 years
Maximum [Member]
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Net Operating Loss Carryforward Period	20 years

[1] For 2013, the amount relates to corrections to the current tax liability account and to deferred tax prior period adjustments.

[2] For 2013 and 2012, the amount relates to increase in valuation allowance on states with net operating loss carryforward periods of 15 to 20 years.

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