Income Taxes - Reconciliation of Expected Income Tax Expense at Federal Statutory Rate of 35 Percent to Company's Applicable Income Tax Expense (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 713	$ 704	$ 2,095	$ 2,091
State income tax, at statutory rates, net of federal tax benefit	60	62	174	163
Tax credits and benefits, net of related expenses	(177)	(186)	(523)	(535)
Tax-exempt income	(51)	(51)	(154)	(154)
Noncontrolling interests	(5)	(5)	(15)	(15)
Other items	(6)	(1)	(36)	16
Applicable income taxes	$ 534	$ 523	$ 1,541	$ 1,566