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, 2020	Mar. 31, 2019
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 302	$ 438
State income tax, at statutory rates, net of federal tax benefit	59	84
Tax credits and benefits, net of related expenses	(102)	(103)
Exam resolutions		(49)
Tax-exempt income	(29)	(32)
Other items	30	40
Applicable income taxes	$ 260	$ 378