Income Taxes - Reconciliation of Expected Income Tax Expense at Federal Statutory Rate of 21 Percent to Company's Applicable Income Tax Expense (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 546	$ 406	$ 1,686	$ 867
State income tax, at statutory rates, net of federal tax benefit	108	75	327	170
Tax credits and benefits, net of related expenses	(91)	(82)	(267)	(280)
Exam resolutions		(47)		(47)
Tax-exempt income	(28)	(29)	(85)	(87)
Other items	29	24	61	48
Applicable income taxes	$ 564	$ 347	$ 1,722	$ 671