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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 533	$ 159	$ 1,140	$ 461
State income tax, at statutory rates, net of federal tax benefit	105	36	219	95
Tax credits and benefits, net of related expenses	(83)	(96)	(176)	(198)
Tax-exempt income	(29)	(29)	(57)	(58)
Other items	25	(6)	32	24
Applicable income taxes	$ 551	$ 64	$ 1,158	$ 324