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, 2018	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds from Sale of Available-for-sale Securities	$ 15,939	$ 143,660
Sales Transactions:
Available-for-sale Securities, Gross Realized Gains	4,719	359
Gross losses	0	(316)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments	4,719	43
Maturities and impairment
Gross Gains	3,383	712
Gross Losses	0	(60)
Gain Losses Maturities And Impairment	3,383	652
Net gains and impairment	$ 8,102	$ 695