Securities - Net Gains and Losses on Trading Account Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Net gain on sales transactions	$ 314	$ 159	$ 716	$ 933
Net mark-to-market gains (losses)	(2)	1	(71)	94
Net gain (loss) on trading account securities	$ 312	$ 160	$ 645	$ 1,027