Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities Held to Maturity (Detail) $ in Thousands	Dec. 31, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	$ 1,596,385	$ 1,778,316
Gross Unrealized Gains	46,217	56,974
Gross Unrealized Losses	(21,563)	(19,314)
Fair Value	1,621,039	1,815,976
U.S. Treasury securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	138,978	139,121
Gross Unrealized Gains	10,505	12,179
Gross Unrealized Losses	0	0
Fair Value	149,483	151,300
U.S. government agency securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	12,859	14,081
Gross Unrealized Gains	271	304
Gross Unrealized Losses	0	0
Fair Value	13,130	14,385
Obligations of states and state agencies [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	194,547	197,440
Gross Unrealized Gains	10,538	9,410
Gross Unrealized Losses	(10)	(412)
Fair Value	205,075	206,438
Municipal bonds [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	310,318	302,578
Gross Unrealized Gains	11,955	10,955
Gross Unrealized Losses	(85)	(278)
Fair Value	322,188	313,255
Obligations of states and political subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	504,865	500,018
Gross Unrealized Gains	22,493	20,365
Gross Unrealized Losses	(95)	(690)
Fair Value	527,263	519,693
Residential mortgage-backed securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	852,289	986,992
Gross Unrealized Gains	11,018	18,233
Gross Unrealized Losses	(8,035)	(6,244)
Fair Value	$ 855,272	$ 998,981
Trust preferred securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Number of available for sale securities (in security) | Security	2	3
Amortized Cost	$ 59,785	$ 98,456
Gross Unrealized Gains	36	167
Gross Unrealized Losses	(13,384)	(12,380)
Fair Value	46,437	86,243
Corporate and other debt securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Amortized Cost	27,609	39,648
Gross Unrealized Gains	1,894	5,726
Gross Unrealized Losses	(49)	0
Fair Value	$ 29,454	$ 45,374