Investment Securities - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments	$ 0	$ 0
Gain Losses Maturities And Impairment	47	15
Available-for-sale Securities, Gross Realized Gains	0	0
Gross Losses	0	0
Gross Gains	47	15
Debt Securities, Held-to-maturity, Restricted	257,600		$ 306,800
Proceeds from Sale of Available-for-sale Securities	0	$ 0
Debt Securities, Available-for-sale, Restricted	$ 865,100		$ 584,800