Income Taxes - Summary of Effective Tax Rates (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Tax provision (benefit) at the U.S. federal statutory income tax rate										$ (303)	$ 270	$ 363
State and local taxes, net of Federal benefit										(14)	12	9
Adjustment with respect to tax for prior periods	[1]									(16)	38	11
Adjustment of tax rate used to value deferred taxes	[2]									(40)	(52)	(5)
Change in valuation allowance	[3]									(90)	(28)	(11)
Uncertain tax positions	[4]									(4)	(2)	(10)
Other non-deductible/non-taxable items	[5]									(4)	(11)	(29)
Other										0	(3)	(3)
Total income tax expense (benefit)		$ (114)	$ (129)	$ (200)	$ (28)	$ 42	$ 87	$ 112	$ (17)	$ (471)	$ 224	$ 325
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										(1.60%)	1.60%	0.90%
Adjustment with respect to tax for prior periods, percent	[1]									(1.80%)	4.90%	1.10%
Adjustment of tax rate used to value deferred taxes, percent	[2]									(4.60%)	(6.70%)	(0.50%)
Change in valuation allowance, percent	[3]									(10.40%)	(3.60%)	(1.10%)
Uncertain tax positions, percent	[4]									(0.50%)	(0.30%)	(1.00%)
Other non-deductible/non-taxable items, percent	[5]									(0.50%)	(1.40%)	(2.80%)
Other, percent										(0.10%)	(0.40%)	(0.30%)
Total income tax expense (benefit), percent										(54.50%)	29.10%	31.30%
State and Local Jurisdiction [Member] | Minimum [Member]
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
NOL Carryforward period											12 years
State and Local Jurisdiction [Member] | Maximum [Member]
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
NOL Carryforward period											20 years

[1] For 2015, the amount relates to an adjustment to a deferred tax balance sheet account as a result of the Federal audit for the 2012 tax year. For 2014, the amount relates to changes in estimates as a result of filing the Federal and State income tax returns and a change in State tax expense as a result of filing amended State tax returns upon the closing of the Federal audits for the 2006 - 2009 tax years. For 2013, 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 2015, the amount mainly relates to the effects of revaluing our deferred tax assets for New York City Tax Reform that was enacted on April 13, 2015. For 2014, the amount primarily relates to the effects of revaluing our deferred tax assets as a result of New York State Tax Reform that was enacted on March 31, 2014.

[3] For 2015, the amount is due to the release of valuation allowance reserves on previously unrecognized State net operating loss carryforwards and temporary differences. For 2014, the amount relates to changes in valuation allowance reserves in States with net operating loss carryforward periods of 12 to 20 years and a release of valuation allowance reserves as a result of filing amended State tax returns upon the closing of the Federal audits for the 2006-2009 tax years. For 2013, the amount relates to changes in valuation allowance in States with net operating loss carryforward periods of 12 to 20 years.

[4] For 2015, 2014 and 2013, the amounts primarily relate to the conclusion of State audits and expiration of State statutes of limitations.

[5] For 2014, the amount primarily relates to tax exempt income and nondeductible penalties. For 2013, the amount includes a change in the estimated deductibility of accrued costs for certain regulatory matters that were accrued during 2011.