Investment Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) Security	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Security	Sep. 30, 2015 USD ($)	Dec. 31, 2015 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	52		52		74
Fair value of investments held to maturity pledged as collateral	$ 860,600,000		$ 860,600,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years			6 years 6 months
Number of security positions in the securities available for sale portfolio in an unrealized loss position (in security) | Security	102		102		291
Amortized cost	$ 1,265,842,000		$ 1,265,842,000		$ 1,516,937,000
Available for sale	1,276,709,000		1,276,709,000		1,506,861,000
Gross unrealized losses	6,357,000		6,357,000		16,312,000
Fair value of securities available for sale pledged as collateral	461,500,000		$ 461,500,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years			9 years 3 months 12 days
Other than temporary impairment losses recognized in earnings	0	$ 0	$ 0	$ 0
Gross gains from sales transactions	0	0	271,000	3,274,000
Amortized cost				25,900,000
Held to Maturity, Amortized Cost	1,845,151,000		1,845,151,000		$ 1,596,385,000
Loss on sales transactions	0	0	0	(947,000)
Trust Preferred Securities Subject to Mandatory Redemption
Investment Securities [Line Items]
Available for sale		8,300,000		8,300,000
Corporate and other debt securities
Investment Securities [Line Items]
Held to Maturity, Amortized Cost		9,800,000		9,800,000
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	$ 2,000,000		$ 2,000,000
Pooled Trust Preferred Securities
Investment Securities [Line Items]
Number of security positions in the securities available for sale portfolio in an unrealized loss position (in security) | Security	2		2
Amortized cost	$ 10,200,000		$ 10,200,000
Available for sale	8,200,000		8,200,000
Pooled Trust Preferred Securities | Investment Grade
Investment Securities [Line Items]
Gross unrealized losses	1,300,000		1,300,000
Four Private Label Mortgage-Backed Securities
Investment Securities [Line Items]
Amortized cost	10,800,000		10,800,000
Available for sale	$ 10,400,000		$ 10,400,000
Pooled Trust Preferred Securities Sold
Investment Securities [Line Items]
Available for sale		$ 2,600,000		$ 2,600,000
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