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		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 409	$ 533	$ 820	$ 1,140
State income tax, at statutory rates, net of federal tax benefit	84	105	168	219
Tax credits and benefits, net of related expenses	(46)	(83)	(152)	(176)
Tax-exempt income	(29)	(29)	(57)	(57)
Other items	(4)	25	32	32
Applicable income taxes	$ 414	$ 551	$ 811	$ 1,158