Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014 Security	Sep. 30, 2013	Sep. 30, 2014 Security	Sep. 30, 2013	Mar. 31, 2014	Dec. 31, 2013 Security	Mar. 31, 2013	Dec. 31, 2012
Investment Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,813,312,000		$ 1,813,312,000			$ 1,731,737,000
Available for sale	758,421,000		758,421,000			829,692,000
Fair value of trading securities	14,186,000		14,186,000			14,264,000
Number of security positions in the securities held to maturity portfolio in an unrealized loss position	86		86			133
Fair value of investments held to maturity pledged as collateral	910,000,000		910,000,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years			7 years 3 months
Amortized Cost	771,375,000		771,375,000			866,904,000
Gross unrealized losses	21,166,000		21,166,000			46,262,000
Gross unrealized losses	21,166,000		21,166,000			46,262,000
Fair value of securities available for sale pledged as collateral	508,000,000		508,000,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years			5 years 3 months
Gains on securities transactions, net	103,000	9,000	102,000	4,008,000
Interest income on trading securities	290,000		871,000	1,100,000
Amortized Cost		941,000
Gross gains, Sales transactions	103,000	1,000	103,000	3,382,000
The credit loss component of cumulative other-than-temporary impairment losses recognized in earnings	9,484,000	32,912,000	9,484,000	32,912,000	9,682,000	9,990,000	33,102,000	33,290,000
Number of security positions in the securities available for sale portfolio in an unrealized loss position	90		90			99
Trust Preferred Securities [Member] | Previously Other-Than-Temporarily Impaired Securities [Member]
Investment Securities [Line Items]
Available for sale	4,700,000		4,700,000
Amortized Cost	5,400,000		5,400,000
Trust Preferred Securities [Member] | Non-Rated [Member]
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position	4		4
Pooled Trust Preferred Securities [Member]
Investment Securities [Line Items]
Available for sale	8,900,000		8,900,000
Amortized Cost	10,900,000		10,900,000
Number of security positions in the securities available for sale portfolio in an unrealized loss position	3		3			2
Pooled Trust Preferred Securities [Member] | Investment Grade [Member]
Investment Securities [Line Items]
Gross unrealized losses	1,300,000		1,300,000
Number of security positions in the securities available for sale portfolio in an unrealized loss position	1		1
Perpetual Preferred Securities [Member]
Investment Securities [Line Items]
Number of security positions in the securities available for sale portfolio in an unrealized loss position	2		2
Three Private Label Mortgage-Backed Securities [Member]
Investment Securities [Line Items]
Available for sale	21,000,000		21,000,000
Number of investment grade private label mortgage-backed securities								5
Amortized Cost	21,200,000		21,200,000
Three Private Label Mortgage-Backed Securities [Member] | Investment Grade [Member]
Investment Securities [Line Items]
Gross unrealized losses	1,300,000		1,300,000
Three Private Label Mortgage-Backed Securities [Member] | Previously Other-Than-Temporarily Impaired Securities [Member]
Investment Securities [Line Items]
Number of investment grade private label mortgage-backed securities								3
Three Private Label Mortgage-Backed Securities [Member] | Previously Other-Than-Temporarily Impaired Securities [Member] | Investment Grade [Member]
Investment Securities [Line Items]
Number of investment grade private label mortgage-backed securities	4		4
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Held to Maturity, Amortized Cost	501,312,000		501,312,000			545,886,000
Available for sale	37,615,000		37,615,000			37,700,000
Amortized Cost	38,238,000		38,238,000			39,112,000
Gross unrealized losses	715,000		715,000			2,163,000
Gross unrealized losses	715,000		715,000			2,163,000
Obligations of States and Political Subdivisions [Member] | Other Than Temporarily Impaired Securities [Member]
Investment Securities [Line Items]
Available for sale	19,200,000		19,200,000
Amortized Cost	18,400,000		18,400,000
Gross unrealized losses	1,700,000		1,700,000			12,900,000
Investment carrying value	538,900,000		538,900,000
Investment carrying value percentage			52.00%
Residential Mortgage-Backed Securities [Member]
Investment Securities [Line Items]
Held to Maturity, Amortized Cost	1,020,852,000		1,020,852,000			886,043,000
Available for sale	459,577,000		459,577,000			508,029,000
Amortized Cost	465,092,000		465,092,000			524,781,000
Gross unrealized losses	9,367,000		9,367,000			20,719,000
Gross unrealized losses	9,367,000		9,367,000			20,719,000
The credit loss component of cumulative other-than-temporary impairment losses recognized in earnings	$ 0		$ 0