Investment Securities - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) Security Securities	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) Security Securities	Jun. 30, 2014 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) Security
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position | Security	108		108			57
Held to Maturity, Amortized Cost	$ 1,720,575		$ 1,720,575			$ 1,778,316
Fair value of investments held to maturity pledged as collateral	$ 974,300		$ 974,300
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years			5 years 7 months
Number of security positions in the securities available for sale portfolio in an unrealized loss position | Security	142		142			96
Amortized cost	$ 817,125		$ 817,125			$ 889,958
Gross unrealized losses	15,566		15,566			12,331
Fair value of securities available for sale pledged as collateral	225,600		$ 225,600
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years			4 years 4 months 47 days
Available for sale	807,574		$ 807,574			886,970
Gross gains, Sales transactions	0	$ 0	3,274	$ 0
Amortized Cost			25,900
Loss on Sale of Investments	0	0	(947)	0
Fair value of trading securities	0		0			$ 14,233
Interest income on trading securities		$ 291	213	$ 581
Corporate and other debt securities
Investment Securities [Line Items]
Held to Maturity, Amortized Cost					$ 9,800
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 | Security	4		4
Pooled Trust Preferred Securities
Investment Securities [Line Items]
Number of security positions in the securities available for sale portfolio in an unrealized loss position	2		2			3
Amortized cost	$ 10,600		$ 10,600
Available for sale	$ 8,600		$ 8,600
Pooled Trust Preferred Securities | Investment Grade
Investment Securities [Line Items]
Number of security positions in the securities available for sale portfolio in an unrealized loss position | Securities	1		1
Gross unrealized losses	$ 1,300		$ 1,300
Three Private Label Mortgage-Backed Securities
Investment Securities [Line Items]
Amortized cost	12,700		12,700
Available for sale	12,600		12,600
Obligations of states and political subdivisions
Investment Securities [Line Items]
Held to Maturity, Amortized Cost	498,128		498,128			$ 500,018
Amortized cost	44,363		44,363			44,500
Gross unrealized losses	993		993			576
Available for sale	43,410		43,410			44,051
Residential mortgage-backed securities
Investment Securities [Line Items]
Held to Maturity, Amortized Cost	965,647		965,647			986,992
Amortized cost	591,342		591,342			643,382
Gross unrealized losses	8,735		8,735			4,960
Available for sale	585,791		585,791			$ 644,276
Pooled Trust Preferred Securities Sold
Investment Securities [Line Items]
Available for sale	2,600		2,600
Trust Preferred Securities Subject to Mandatory Redemption
Investment Securities [Line Items]
Available for sale	$ 8,300		$ 8,300