Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	$ 1,778,316us-gaap_HeldToMaturitySecurities	$ 1,731,737us-gaap_HeldToMaturitySecurities
Gross Unrealized Gains	56,974	30,962
Gross Unrealized Losses	(19,314)	(51,272)
Fair Value	1,815,976	1,711,427
U.S. Treasury securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	139,121	139,255
Gross Unrealized Gains	12,179	5,567
Gross Unrealized Losses	0	(515)
Fair Value	151,300	144,307
U.S. government agency securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	14,081	4,427
Gross Unrealized Gains	304	0
Gross Unrealized Losses	0	(62)
Fair Value	14,385	4,365
Obligations of states and state agencies [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	197,440	192,653
Gross Unrealized Gains	9,410	1,944
Gross Unrealized Losses	(412)	(5,473)
Fair Value	206,438	189,124
Municipal bonds [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	302,578	353,233
Gross Unrealized Gains	10,955	6,053
Gross Unrealized Losses	(278)	(5,259)
Fair Value	313,255	354,027
Obligations of states and political subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	500,018	545,886
Gross Unrealized Gains	20,365	7,997
Gross Unrealized Losses	(690)	(10,732)
Fair Value	519,693	543,151
Residential mortgage-backed securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	986,992	886,043
Gross Unrealized Gains	18,233	12,609
Gross Unrealized Losses	(6,244)	(27,631)
Fair Value	998,981	871,021
Trust preferred securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	98,456	103,458
Gross Unrealized Gains	167	363
Gross Unrealized Losses	(12,380)	(12,332)
Fair Value	86,243	91,489
Corporate and other debt securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	39,648	52,668
Gross Unrealized Gains	5,726	4,426
Gross Unrealized Losses	0	0
Fair Value	$ 45,374	$ 57,094

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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