Investment Securities - Proceeds from Sale, Gross Gains (Losses) Realized on Sales, Maturities and Other-Than-Temporary Impairment Charges Related to Securities Available for Sale (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Proceeds from Sale of Available-for-sale Securities	$ 0	$ 15,939
Sales Transactions:
Available-for-sale Securities, Gross Realized Gains	0	4,719
Gross losses	0	0
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments	0	4,719
Maturities and impairment
Gross Gains	6	3,383
Gross Losses	0	0
Gain Losses Maturities And Impairment	6	3,383
Net gains and impairment	$ 6	$ 8,102