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		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 727	$ 709	$ 1,394	$ 1,382
State income tax, at statutory rates, net of federal tax benefit	54	63	117	114
Tax credits and benefits, net of related expenses	(174)	(182)	(340)	(346)
Tax-exempt income	(49)	(51)	(99)	(103)
Noncontrolling interests	(5)	(6)	(10)	(10)
Other items	(11)	(5)	(16)	(30)
Applicable income taxes	$ 542	$ 528	$ 1,046	$ 1,007