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	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Proceeds from Sale of Available-for-sale Securities	$ 143,660	$ 55,744
Sales Transactions:
Gross gains	359	304
Gross losses	(316)	(276)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments	43	28
Maturities and impairment
Gross Gains	712	0
Gross Losses	(60)	0
Gain Losses Maturities And Impairment	652	0
Net gains and impairment	$ 695	$ 28