Investment Securities - Additional Information (Detail)	6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) secuirty	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Trading securities	$ 0
Available-for-sale securities, amortized cost basis	825,488,000		$ 807,761,000
Securities pledged as collateral	571,600,000		562,500,000
Proceeds from sale of investment securities	351,600,000	$ 101,300,000
Gains from sale of Available-for-sale securities	1,100,000	900,000
Losses from sale of Available-for-sale securities (less than during the current quarter end)	100,000	$ 0
Unamortized premiums	15,500,000		18,000,000
Unaccreted discounts	800,000		400,000
Owned investment securities	530,700,000
Total unrealized losses on securities	$ 10,000,000
Number of securities, below investment grade | secuirty	1
Available-for-sale securities, below investment grade, fair value	$ 1,000,000
Weighted average duration of MBS portfolio	5 years 15 days
Mortgage-Backed Securities (MBS)
Schedule of Available-for-sale Securities [Line Items]
OTTI on evaluation of securities	$ 0
Mutual Fund
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	600,000		600,000
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities transfers to held to maturity unrealized gains	$ 1,900,000		$ 2,200,000