Investment Securities - Additional Information (Detail)	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) secuirty	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Trading securities	$ 0
Available-for-sale securities, amortized cost basis	816,114,000		$ 807,761,000
Securities pledged as collateral	737,300,000		562,500,000
Proceeds from sale of investment securities	415,500,000	$ 155,800,000
Gains from sale of Available-for-sale securities	1,900,000	1,900,000
Losses from sale of Available-for-sale securities (less than during the current quarter end)	100,000	$ 0
Unamortized premiums	14,900,000		18,000,000
Unaccreted discounts	1,100,000		400,000
Owned investment securities	511,300,000
Total unrealized losses on securities	$ 8,800,000
Number of securities, below investment grade | secuirty	1
Available-for-sale securities, below investment grade, fair value	$ 800,000
Weighted average duration of MBS portfolio	5 years 1 month 6 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	$ 1,800,000		$ 2,200,000