Investment Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Security	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Security	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Security
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position (in security) | Security	118		118		132
Fair value of investments held to maturity pledged as collateral	$ 978,700,000		$ 978,700,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years			7 years 4 months
Number of security positions in the securities available for sale portfolio in an unrealized loss position (in security) | Security	253		253		298
Amortized cost	$ 1,458,572,000		$ 1,458,572,000		$ 1,315,723,000
Available for sale	1,447,737,000		1,447,737,000		1,297,373,000
Gross unrealized losses	16,901,000		16,901,000		$ 22,416,000
Fair value of securities available for sale pledged as collateral	692,200,000		$ 692,200,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years			8 years 2 months
Other than temporary impairment losses recognized in earnings	0	$ 0	$ 0	$ 0
Previously Other-Than-Temporarily Impaired Securities | Trust Preferred Securities Subject to Mandatory Redemption
Investment Securities [Line Items]
Amortized cost	2,800,000		2,800,000
Available for sale	2,100,000		2,100,000
Four Private Label Mortgage-Backed Securities
Investment Securities [Line Items]
Amortized cost	8,600,000		8,600,000
Available for sale	$ 7,900,000		$ 7,900,000
Single Issuer Trust Preferred Securities | Non-Rated
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position (in security) | Security	4		4