Investment Securities - Unrealized Losses and Fair Value of Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	$ 670,249	$ 365,182
Less than Twelve Months, Unrealized Losses	(14,695)	(2,702)
More than Twelve Months, Fair Value	625,029	663,238
More than Twelve Months, Unrealized Losses	(35,834)	(27,788)
Total, Fair Value	1,295,278	1,028,420
Total, Unrealized Losses	(50,529)	(30,490)
U.S. government agency securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	6,674
Less than Twelve Months, Unrealized Losses	(173)
More than Twelve Months, Fair Value	0
More than Twelve Months, Unrealized Losses	0
Total, Fair Value	6,674
Total, Unrealized Losses	(173)
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	120,794	6,342
Less than Twelve Months, Unrealized Losses	(1,759)	(50)
More than Twelve Months, Fair Value	50,493	53,034
More than Twelve Months, Unrealized Losses	(2,982)	(1,603)
Total, Fair Value	171,287	59,376
Total, Unrealized Losses	(4,741)	(1,653)
Municipal bonds
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	86,817	4,644
Less than Twelve Months, Unrealized Losses	(1,385)	(25)
More than Twelve Months, Fair Value	534	561
More than Twelve Months, Unrealized Losses	(30)	(3)
Total, Fair Value	87,351	5,205
Total, Unrealized Losses	(1,415)	(28)
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	207,611	10,986
Less than Twelve Months, Unrealized Losses	(3,144)	(75)
More than Twelve Months, Fair Value	51,027	53,595
More than Twelve Months, Unrealized Losses	(3,012)	(1,606)
Total, Fair Value	258,638	64,581
Total, Unrealized Losses	(6,156)	(1,681)
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	448,658	344,216
Less than Twelve Months, Unrealized Losses	(10,831)	(2,357)
More than Twelve Months, Fair Value	544,329	570,969
More than Twelve Months, Unrealized Losses	(26,021)	(16,386)
Total, Fair Value	992,987	915,185
Total, Unrealized Losses	(36,852)	(18,743)
Trust preferred securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	95	0
Less than Twelve Months, Unrealized Losses	(258)	0
More than Twelve Months, Fair Value	29,673	38,674
More than Twelve Months, Unrealized Losses	(6,801)	(9,796)
Total, Fair Value	29,768	38,674
Total, Unrealized Losses	(7,059)	(9,796)
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	7,211	9,980
Less than Twelve Months, Unrealized Losses	(289)	(270)
More than Twelve Months, Fair Value	0	0
More than Twelve Months, Unrealized Losses	0	0
Total, Fair Value	7,211	9,980
Total, Unrealized Losses	$ (289)	$ (270)