Investment Securities - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments	$ 0	$ 4,719
Gain Losses Maturities And Impairment	6	3,383
Available-for-sale Securities, Gross Realized Gains	0	4,719
Gross Losses	0	0
Gross Gains	6	3,383
Debt Securities, Held-to-maturity, Restricted	333,600		$ 250,300
Proceeds from Sale of Available-for-sale Securities	0	$ 15,939
Debt Securities, Available-for-sale, Restricted	$ 589,400		$ 636,300