Basis of Presentation - Adoptions of New Accounting Guidance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in other income, adoption of accounting guidance		$ 224		$ 446
Increase in income tax expense, adoption of accounting guidance		$ 229		$ 456
Increase in the effective income tax rate		2.00%		2.00%
Income tax credits and adjustments	$ 381	$ 384	$ 758	$ 763
Amortization of investments reported in income tax expense	281	$ 267	555	$ 531
Carrying value of investments in affordable housing projects	7,100		7,100		$ 7,300
Commitments related to affordable housing investments	$ 1,700		$ 1,700		1,800
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle					$ 0	$ (321)