Investment Securities - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) secuirty	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Trading securities	$ 0
Available-for-sale securities, amortized cost basis	800,612,000		$ 807,761,000
Securities pledged as collateral	615,800,000		562,500,000
Proceeds from sale of investment securities	263,000,000	$ 38,900,000
Gains from sale of Available-for-sale securities	$ 300,000	300,000
Security sold | secuirty	1
Losses from sale of Available-for-sale securities	$ 0	$ 0
Unamortized premiums	16,300,000		18,000,000
Unaccreted discounts	600,000		400,000
Owned investment securities	612,300,000
Total unrealized losses on securities	$ 14,700,000
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 4 months 2 days
Mortgage-Backed Securities (MBS)
Schedule of Available-for-sale Securities [Line Items]
OTTI on evaluation of securities	$ 0		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]
Securities transferred from available-for-sale to held-to-maturity, net unrealized gain	$ 2,100,000		$ 2,200,000