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
	Mar. 31, 2016	Mar. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 667	$ 673
State income tax, at statutory rates, net of federal tax benefit	63	51
Tax credits and benefits, net of related expenses	(166)	(164)
Tax-exempt income	(50)	(52)
Noncontrolling interests	(5)	(4)
Other items	(5)	(25)
Applicable income taxes	$ 504	$ 479