Investment Securities - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) Security	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Security	Sep. 30, 2014 USD ($)	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	79		79			57
Held to Maturity, Amortized Cost	$ 1,637,310		$ 1,637,310			$ 1,778,316
Fair value of investments held to maturity pledged as collateral	$ 1,000,000		$ 1,000,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years			5 years 10 months 25 days
Number of security positions in the securities available for sale portfolio in an unrealized loss position | Security	94		94			96
Amortized cost	$ 799,264		$ 799,264			$ 889,958
Gross unrealized losses	10,849		10,849			12,331
Fair value of securities available for sale pledged as collateral	242,500		$ 242,500
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years			4 years 10 months 2 days
Available for sale	797,389		$ 797,389			886,970
Gross gains, Sales transactions	0	$ 103	3,274	$ 103
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		$ 290	213	$ 871
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	$ 2,200		$ 2,200
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 | Security	2		2			3
Amortized cost	$ 10,500		$ 10,500
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 | Security	1		1
Gross unrealized losses	$ 1,400		$ 1,400
Three Private Label Mortgage-Backed Securities
Investment Securities [Line Items]
Amortized cost	12,300		12,300
Available for sale	12,200		12,200
Obligations of states and political subdivisions
Investment Securities [Line Items]
Held to Maturity, Amortized Cost	511,788		511,788			$ 500,018
Amortized cost	44,195		44,195			44,500
Gross unrealized losses	808		808			576
Available for sale	43,469		43,469			44,051
Residential mortgage-backed securities
Investment Securities [Line Items]
Held to Maturity, Amortized Cost	889,146		889,146			986,992
Amortized cost	569,182		569,182			643,382
Gross unrealized losses	5,397		5,397			4,960
Available for sale	569,650		569,650			$ 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