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, 2017	Mar. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 695	$ 667
State income tax, at statutory rates, net of federal tax benefit	63	63
Tax credits and benefits, net of related expenses	(193)	(166)
Tax-exempt income	(49)	(50)
Noncontrolling interests	(5)	(5)
Other items	(12)	(5)
Applicable income taxes	$ 499	$ 504