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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 159	$ 478	$ 461	$ 916
State income tax, at statutory rates, net of federal tax benefit	36	94	95	178
Tax credits and benefits, net of related expenses	(96)	(107)	(198)	(210)
Exam resolutions				(49)
Tax-exempt income	(29)	(31)	(58)	(63)
Other items	(6)	15	24	55
Applicable income taxes	$ 64	$ 449	$ 324	$ 827