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Income Taxes - Summary of Effective Tax Rates (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]        
Tax expense (benefit) at the U.S. federal statutory income tax rate $ 141 $ (858) $ 300 $ (1,136)
State and local taxes, net of Federal benefit 2 (42) 5 (45)
Adjustment with respect to tax for prior periods 14 [1] (43) [1] 4 [1] (44) [1]
Adjustment of tax rate used to value deferred taxes (6) 0 (11) (7)
Change in valuation allowance reserves (15) [2] 3 [2] (5) [2] 10 [2]
Uncertain tax adjustments     (5) [3] (5) [3]
Other non-deductible/non-taxable items (5) [4] (2) [4] (5) [4] (3) [4]
Impact of foreign operations 2 0 2 0
Other 0 3 0 3
Total income tax expense (benefit) $ 133 $ (939) $ 285 $ (1,227)
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]        
Tax expense (benefit) at the U.S. federal statutory income tax rate, percent 35.00% (35.00%) 35.00% (35.00%)
State and local taxes, net of Federal benefit, percent 0.50% (1.70%) 0.60% (1.40%)
Adjustment with respect to tax for prior periods 3.50% [1] (1.70%) [1] 0.50% [1] (1.40%) [1]
Adjustment of tax rate used to value deferred taxes (1.60%) 0.00% (1.30%) (0.20%)
Change in valuation allowance reserves, percent (3.80%) [2] 0.10% [2] (0.60%) [2] 0.30% [2]
Uncertain tax adjustments     (0.60%) [3] (0.10%) [3]
Other non-deductible/non-taxable items, percent (1.20%) [4] (0.10%) [4] (0.60%) [4] (0.10%) [4]
Impact of foreign operations 0.50% 0.00% 0.10% 0.00%
Other, percent 0.00% 0.10% 0.00% 0.10%
Total income tax expense (benefit), percent 32.90% (38.30%) 33.10% (37.80%)
[1] For 2013 and 2012, the amounts relate to corrections to current and deferred tax balance sheet accounts.
[2] For 2013 and 2012, the amounts relate to changes in valuation allowance on states with net operating loss carryforward periods of 12 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.
[4] For 2013, the amount relates to a change in the estimated deductibility of accrued costs for certain regulatory matters and a correction to share-based compensation deferred balances.