Investment Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 security	Dec. 31, 2017 Security	Dec. 31, 2016 USD ($) security Security	Dec. 31, 2015 USD ($)
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position | security		152		132
Fair value of investments held to maturity pledged as collateral	$ 1,100,000,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years	7 years 3 months
Number of security positions in the securities available for sale portfolio in an unrealized loss position | security		327		298
Fair value of securities available for sale pledged as collateral	$ 775,600,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years	8 years
Amortized cost	$ 1,511,034,000			$ 1,315,723,000
Available for sale	1,493,905,000			1,297,373,000
Single Issuer Trust Preferred Security | Non-Rated
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position | Security			4
Trust preferred securities
Investment Securities [Line Items]
Other-than-temporary impairment	0			0	$ 0
Three Private Label Mortgage-Backed Securities
Investment Securities [Line Items]
Number of previously impaired securities (in security) | security		4
Other-than-temporarily impaired mortgage-backed securities, amortized cost	7,900,000
Other-than-temporarily impaired mortgage-backed securities, fair value	7,400,000
Trust preferred securities
Investment Securities [Line Items]
Amortized cost	3,726,000			10,164,000
Available for sale	3,214,000			$ 8,009,000
Number of available for sale securities		1	1	2
Number of previously impaired securities (in security) | security		1
Obligations of states and political subdivisions
Investment Securities [Line Items]
Amortized cost	113,187,000			$ 120,741,000
Available for sale	$ 112,884,000			$ 119,767,000
Obligations of states and political subdivisions | Other Than Temporarily Impaired Securities
Investment Securities [Line Items]
Investment carrying value percentage	49.00%
Investment carrying value	$ 578,800,000
Amortized cost	20,100,000
Available for sale	$ 20,800,000