Income Taxes - Effective tax rates (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income tax reconciliation
Tax expense (benefit) at the U.S. federal statutory income tax rate										$ 76	$ 196	$ 104
State and local taxes, net of federal benefit										10	20	15
Adjustment of tax rate used to value deferred taxes	[1]									4	47	63
Other non-deductible / non-taxable items	[2]									21	1	0
Items affecting prior periods	[3]									(2)	(7)	(29)
Uncertain tax positions	[4]									(4)	4	(192)
Low income housing tax credit investments										(17)	(26)	(26)
Change in valuation allowances reserve	[5]									0	(5)	10
Other										1	0	(1)
Total income tax expense (benefit)		$ (11)	$ 26	$ (5)	$ 79	$ (55)	$ 111	$ 98	$ 76	$ 89	$ 230	$ (56)
Effective income tax reconciliation
Tax expense (benefit) at the U.S. federal statutory income tax rate, percentage										35.00%	35.00%	35.00%
State and local taxes, net of federal benefit, percentage										4.60%	3.60%	5.00%
Adjustment of tax rate used to value deferred taxes, percentage	[1]									1.80%	8.40%	21.10%
Other non-deductible / non-taxable items, percentage	[2]									9.60%	0.20%	0.00%
Items affecting prior periods, percentage	[3]									(0.90%)	(1.30%)	(9.70%)
Uncertain tax positions, percentage	[4]									(1.80%)	0.70%	(64.40%)
Low income housing tax credit investments, percentage										(7.80%)	(4.60%)	(8.70%)
Change in valuation allowances reserves, percentage	[5]									0.00%	(0.90%)	3.40%
Other, percentage										0.50%	0.00%	(0.30%)
Effective tax rate, percentage										40.80%	41.10%	(18.80%)

[1] 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 mainly relates to the effects of revaluing our deferred tax assets for New York State Tax Reform that was enacted on March 31, 2014.

[2] For 2016, the amount mainly relates to the accrual of non-deductible penalties.

[3] 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.

[4] For 2014, the amount mainly reflects the resolution and settlement with taxation authorities of certain significant State and local tax audits during 2014 which is discussed further below.

[5] For 2014, the amount relates to the establishment of a valuation allowance against our deferred tax assets as a result of New York State Tax Reform that was enacted on March 31, 2014.