Fair Value Disclosures (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 2,483,375,000		$ 2,577,901,000
Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Assets	2,506,311,000		2,603,816,000
Total Liabilities	54,712,000		72,126,000
Designated As Hedging Instrument [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivatives, Assets	1,457,000	[1]	878,000	[1]
Derivative, Liabilities	36,209,000	[2]	51,172,000	[2]
Not Designated As Hedging Instruments [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivatives, Assets	21,479,000	[1]	25,037,000	[1]
Derivative, Liabilities	18,503,000	[2]	20,954,000	[2]
U.S. Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	85,193,000		114,408,000
U.S. Government Agencies [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	85,193,000		114,408,000
Obligations Of States And Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	401,278,000		435,777,000
Obligations Of States And Political Subdivisions [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	401,278,000		435,777,000
Agency Residential Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	1,884,810,000		1,880,562,000
Agency Residential Mortgage-Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	1,884,810,000		1,880,562,000
Non-Agency Residential Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	23,596,000		27,450,000
Non-Agency Residential Mortgage-Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	23,596,000		27,450,000
Commercial Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	10,492,000		40,380,000
Commercial Mortgage Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	10,492,000		40,380,000
Other Structured Financial Products [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	11,444,000		9,550,000
Other Structured Financial Products [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	11,444,000		9,550,000
Other Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	42,829,000		45,255,000
Other Debt Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	42,829,000		45,255,000
Other Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	23,733,000		24,519,000
Other Equity Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	23,733,000		24,519,000
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	20,912,000		21,266,000
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Assets	20,912,000		21,266,000
Total Liabilities	0		0
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | Designated As Hedging Instrument [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivatives, Assets	0	[1]	0	[1]
Derivative, Liabilities	0	[2]	0	[2]
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | Not Designated As Hedging Instruments [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivatives, Assets	0	[1]	0	[1]
Derivative, Liabilities	0	[2]	0	[2]
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | U.S. Government Agencies [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | Obligations Of States And Political Subdivisions [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | Agency Residential Mortgage-Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | Non-Agency Residential Mortgage-Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | Commercial Mortgage Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | Other Structured Financial Products [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | Other Debt Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Quoted Prices In Active Markets For Identical Instruments (Level 1) [Member] | Other Equity Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	20,912,000		21,266,000
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	2,425,176,000		2,517,129,000
Significant Other Observable Inputs (Level 2) [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Assets	2,448,112,000		2,543,044,000
Total Liabilities	54,712,000		72,126,000
Significant Other Observable Inputs (Level 2) [Member] | Designated As Hedging Instrument [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivatives, Assets	1,457,000	[1]	878,000	[1]
Derivative, Liabilities	36,209,000	[2]	51,172,000	[2]
Significant Other Observable Inputs (Level 2) [Member] | Not Designated As Hedging Instruments [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivatives, Assets	21,479,000	[1]	25,037,000	[1]
Derivative, Liabilities	18,503,000	[2]	20,954,000	[2]
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Agencies [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	85,193,000		114,408,000
Significant Other Observable Inputs (Level 2) [Member] | Obligations Of States And Political Subdivisions [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	401,278,000		435,777,000
Significant Other Observable Inputs (Level 2) [Member] | Agency Residential Mortgage-Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	1,884,810,000		1,880,562,000
Significant Other Observable Inputs (Level 2) [Member] | Non-Agency Residential Mortgage-Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	574,000		650,000
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	10,492,000		40,380,000
Significant Other Observable Inputs (Level 2) [Member] | Other Structured Financial Products [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Significant Other Observable Inputs (Level 2) [Member] | Other Debt Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	42,829,000		45,255,000
Significant Other Observable Inputs (Level 2) [Member] | Other Equity Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		97,000
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	37,287,000		39,506,000
Significant Unobservable Inputs (Level 3) [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total Assets	37,287,000		39,506,000
Total Liabilities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Designated As Hedging Instrument [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivatives, Assets	0	[1]	0	[1]
Derivative, Liabilities	0	[2]	0	[2]
Significant Unobservable Inputs (Level 3) [Member] | Not Designated As Hedging Instruments [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivatives, Assets	0	[1]	0	[1]
Derivative, Liabilities	0	[2]	0	[2]
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government Agencies [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Obligations Of States And Political Subdivisions [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Agency Residential Mortgage-Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Non-Agency Residential Mortgage-Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	23,022,000		26,800,000
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Other Structured Financial Products [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	11,444,000		9,550,000
Significant Unobservable Inputs (Level 3) [Member] | Other Debt Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Other Equity Securities [Member] | Measured On Recurring Basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 2,821,000		$ 3,156,000

[1] Included in Other assets

[2] Included in Other liabilities