Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,825,819us-gaap_HeldToMaturitySecurities	$ 1,778,316us-gaap_HeldToMaturitySecurities
Gross Unrealized Gains	59,330	56,974
Gross Unrealized Losses	(16,524)	(19,314)
Fair Value	1,868,625	1,815,976
U.S. Treasury Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	139,087	139,121
Gross Unrealized Gains	14,589	12,179
Gross Unrealized Losses	0	0
Fair Value	153,676	151,300
U.S. Government Agency Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	13,630	14,081
Gross Unrealized Gains	410	304
Gross Unrealized Losses	0	0
Fair Value	14,040	14,385
Obligations of States and State Agencies [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	196,849	197,440
Gross Unrealized Gains	10,245	9,410
Gross Unrealized Losses	(152)	(412)
Fair Value	206,942	206,438
Municipal Bonds [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	293,908	302,578
Gross Unrealized Gains	12,152	10,955
Gross Unrealized Losses	(86)	(278)
Fair Value	305,974	313,255
Obligations of States and Political Subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	490,757	500,018
Gross Unrealized Gains	22,397	20,365
Gross Unrealized Losses	(238)	(690)
Fair Value	512,916	519,693
Residential Mortgage-Backed Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	1,059,278	986,992
Gross Unrealized Gains	19,205	18,233
Gross Unrealized Losses	(3,804)	(6,244)
Fair Value	1,074,679	998,981
Trust Preferred Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	98,458	98,456
Gross Unrealized Gains	104	167
Gross Unrealized Losses	(12,482)	(12,380)
Fair Value	86,080	86,243
Corporate and Other Debt Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	24,609	39,648
Gross Unrealized Gains	2,625	5,726
Gross Unrealized Losses	0	0
Fair Value	$ 27,234	$ 45,374

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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