Investment Securities - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) Security	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) Security	Jun. 30, 2015 USD ($)	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	74		74		74
Fair value of investments held to maturity pledged as collateral	$ 916,500		$ 916,500
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years			6 years 1 month
Number of security positions in the securities available for sale portfolio in an unrealized loss position (in security) | Security	117		117		291
Held to Maturity, Amortized Cost	$ 1,765,220		$ 1,765,220		$ 1,596,385
Amortized cost	1,183,648		1,183,648		1,516,937
Gross unrealized losses	9,370		9,370		16,312
Fair value of securities available for sale pledged as collateral	401,300		$ 401,300
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years			9 years 8 months 28 days
Available for sale	1,186,341		$ 1,186,341		1,506,861
Gross gains, Sales transactions	0	$ 0	271	$ 3,274
Amortized Cost				25,900
Loss on Sale of Investments	$ 0	0	$ 0	(947)
Trust Preferred Securities Subject to Mandatory Redemption
Investment Securities [Line Items]
Available for sale		8,300		8,300
Corporate and other debt securities
Investment Securities [Line Items]
Held to Maturity, Amortized Cost		9,800		9,800
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	$ 10,300		$ 10,300
Available for sale	8,300		8,300
Pooled Trust Preferred Securities | Investment Grade
Investment Securities [Line Items]
Gross unrealized losses	1,400		1,400
Four Private Label Mortgage-Backed Securities
Investment Securities [Line Items]
Amortized cost	11,300		11,300
Available for sale	11,000		11,000
Obligations of states and political subdivisions
Investment Securities [Line Items]
Held to Maturity, Amortized Cost	565,509		565,509		504,865
Amortized cost	123,901		123,901		125,002
Gross unrealized losses	410		410		441
Available for sale	125,746		125,746		124,966
Residential mortgage-backed securities
Investment Securities [Line Items]
Held to Maturity, Amortized Cost	957,377		957,377		852,289
Amortized cost	873,480		873,480		701,764
Gross unrealized losses	4,627		4,627		8,684
Available for sale	876,153		876,153		$ 696,428
Pooled Trust Preferred Securities Sold
Investment Securities [Line Items]
Available for sale		$ 2,600		$ 2,600
Trust Preferred Securities Subject to Mandatory Redemption | Previously Other-Than-Temporarily Impaired Securities
Investment Securities [Line Items]
Amortized cost	2,800		2,800
Available for sale	$ 2,100		$ 2,100
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