Investment Securities - Amortized Cost, Fair Value and Yield by Maturity Date (Parenthetical) (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020		Jun. 30, 2020		Dec. 31, 2019
Contractual Maturities of Investment Securities [Abstract]
Weighted-average maturity of available-for-sale investment securities	3 years	[1]			4 years 2 months 12 days
Weighted-average yield of available-for-sale investment securities	2.05%	[1],[2]	2.05%	[1],[2]	2.38%
Federal statutory rate	21.00%		21.00%		21.00%

[1] The weighted-average maturity of total available-for-sale investment securities was 4.2 years at December 31, 2019, with a corresponding weighted-average yield of 2.38 percent.

[2] Weighted-average yields for obligations of state and political subdivisions are presented on a fully-taxable equivalent basis based on a federal income tax rate of 21 percent. Yields on investment securities are computed based on amortized cost balances.