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
	Mar. 31, 2021	Mar. 31, 2020
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 607	$ 302
State income tax, at statutory rates, net of federal tax benefit	114	59
Tax credits and benefits, net of related expenses	(93)	(102)
Tax-exempt income	(28)	(29)
Other items	7	30
Applicable income taxes	$ 607	$ 260