Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,637,310	$ 1,778,316
Gross Unrealized Gains	51,879	56,974
Gross Unrealized Losses	(18,238)	(19,314)
Held to maturity, fair value	1,670,951	1,815,976
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	139,014	139,121
Gross Unrealized Gains	13,499	12,179
Gross Unrealized Losses	0	0
Held to maturity, fair value	152,513	151,300
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	12,973	14,081
Gross Unrealized Gains	427	304
Gross Unrealized Losses	0	0
Held to maturity, fair value	13,400	14,385
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	195,143	197,440
Gross Unrealized Gains	9,471	9,410
Gross Unrealized Losses	(12)	(412)
Held to maturity, fair value	204,602	206,438
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	316,645	302,578
Gross Unrealized Gains	10,678	10,955
Gross Unrealized Losses	(309)	(278)
Held to maturity, fair value	327,014	313,255
Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	511,788	500,018
Gross Unrealized Gains	20,149	20,365
Gross Unrealized Losses	(321)	(690)
Held to maturity, fair value	531,616	519,693
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	889,146	986,992
Gross Unrealized Gains	15,530	18,233
Gross Unrealized Losses	(4,900)	(6,244)
Held to maturity, fair value	899,776	998,981
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	59,780	98,456
Gross Unrealized Gains	48	167
Gross Unrealized Losses	(13,017)	(12,380)
Held to maturity, fair value	46,811	86,243
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Held to Maturity, Amortized Cost	24,609	39,648
Gross Unrealized Gains	2,226	5,726
Gross Unrealized Losses	0	0
Held to maturity, fair value	$ 26,835	$ 45,374