Investment Securities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, amortized cost	$ 848,434,000	$ 807,761,000
Investment securities, available for sale (book value $848,434 and $807,761 at December 31, 2017 and 2016, respectively)	838,122,000	794,543,000
Securities pledged as collateral	688,200,000	562,500,000
Proceeds from sale of investment securities	457,000,000	201,800,000	$ 192,800,000
Gains from sale of Available-for-sale securities	2,100,000	2,400,000	1,500,000
Losses from sale of Available-for-sale securities (less than)	100,000	100,000	$ 100,000
Unamortized premiums	14,100,000	18,000,000
Unaccreted discounts	1,300,000	400,000
Owned investment securities	668,100,000
Total unrealized losses on securities	11,800,000
Available-For-Sale Securities, Below Investment Grade, Fair Value	$ 800,000
Weighted average duration of MBS portfolio	5 years 2 months 8 days
Mortgage-Backed Securities (MBS)
Schedule of Available-for-sale Securities [Line Items]
OTTI on evaluation of securities	$ 0	0
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	1,600,000	$ 2,200,000
Mutual Fund
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, amortized cost	600,000
Investment securities, available for sale (book value $848,434 and $807,761 at December 31, 2017 and 2016, respectively)	$ 600,000