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, 2018	Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds from Sale of Available-for-sale Securities	$ 15,939	$ 103,618
Sales Transactions:
Available-for-sale Securities, Gross Realized Gains	4,719	0
Gross losses	0	(49)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments	4,719	(49)
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	$ 603