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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 755	$ 729	$ 2,172	$ 2,123
State income tax, at statutory rates, net of federal tax benefit	71	53	201	170
Tax credits and benefits, net of related expenses	(187)	(183)	(577)	(523)
Tax-exempt income	(50)	(49)	(150)	(148)
Noncontrolling interests	(2)	(5)	(11)	(15)
Other items	2	21	4	5
Applicable income taxes	$ 589	$ 566	$ 1,639	$ 1,612