Income Taxes - Effective tax rates (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Income tax reconciliation
Tax expense (benefit) at the U.S. federal statutory income tax rate										$ 367		$ 76		$ 196
State and local taxes, net of federal benefit										28		10		20
Adjustment of Federal tax rate used to value deferred taxes		$ 865								865	[1]	0	[1]	0	[1]
Adjustment of State tax rate used to value deferred taxes	[2]									(15)		4		47
Other non-deductible / non-taxable items	[3]									1		21		1
Items affecting prior periods	[4]									6		(2)		(7)
Uncertain tax positions										0		(4)		4
Change in valuation allowances reserve										0		0		(5)
Stock based compensation	[5]									(8)		0		0
Other										1		1		0
Total income tax expense (benefit)		$ 907	$ 61	$ 108	$ 152	$ (11)	$ 26	$ (5)	$ 79	1,228		$ 89		$ 230
Out of period adjustment to deferred tax assets										$ 9
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										2.70%		4.60%		3.60%
Adjustment of Federal tax rate used to value deferred taxes, percentage	[1]									82.40%		0.00%		0.00%
Adjustment of State tax rate used to value deferred taxes, percentage	[2]									(1.40%)		1.80%		8.40%
Other non-deductible / non-taxable items, percentage	[3]									0.10%		9.60%		0.20%
Items affecting prior periods, percentage	[4]									0.60%		(0.90%)		(1.30%)
Uncertain tax positions, percentage										0.00%		(1.80%)		0.70%
Low income housing and other tax credit investments										$ (17)		$ (17)		$ (26)
Low income housing and other tax credit investments, percentage										(1.60%)		(7.80%)		(4.60%)
Change in valuation allowances reserves, percentage										0.00%		0.00%		(0.90%)
Stock based compensation, percent	[5]									(0.80%)		0.00%		0.00%
Other, percentage										0.10%		0.50%		0.00%
Effective tax rate, percentage										117.10%		40.80%		41.10%

[1] For 2017, the amount relates to the effects of revaluing our net deferred tax asset for new Tax Legislation that was enacted on December 22, 2017.

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

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

[4] For 2017, the amount relates to the impact of adjustments associated with filing the 2016 State income tax returns and changes in tax credits as a result of filing the 2016 Federal income tax return.

[5] As discussed more fully in Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," beginning January 1, 2017, all excess tax benefits and tax deficiencies for share-based payment awards are recorded within income tax expense in the consolidated statement of income (loss).