Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	$ 640,767	$ 374,643
Less than Twelve Months, Unrealized Losses	(34,020)	(4,118)
More than Twelve Months, Fair Value	61,543	63,882
More than Twelve Months, Unrealized Losses	(9,454)	(15,361)
Total, Fair Value	702,310	438,525
Total, Unrealized Losses	(43,474)	(19,479)
U.S. Treasury securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	89,786	97,625
Less than Twelve Months, Unrealized Losses	(10,053)	(2,218)
More than Twelve Months, Fair Value
More than Twelve Months, Unrealized Losses
Total, Fair Value	89,786	97,625
Total, Unrealized Losses	(10,053)	(2,218)
Residential mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	442,850	269,895
Less than Twelve Months, Unrealized Losses	(18,946)	(1,256)
More than Twelve Months, Fair Value	13,566	21,089
More than Twelve Months, Unrealized Losses	(1,866)	(2,103)
Total, Fair Value	456,416	290,984
Total, Unrealized Losses	(20,812)	(3,359)
Trust preferred securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	446	760
Less than Twelve Months, Unrealized Losses	(10)	(511)
More than Twelve Months, Fair Value	32,969	27,865
More than Twelve Months, Unrealized Losses	(5,638)	(11,228)
Total, Fair Value	33,415	28,625
Total, Unrealized Losses	(5,648)	(11,739)
Corporate and other debt securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	54,149	5,394
Less than Twelve Months, Unrealized Losses	(2,192)	(58)
More than Twelve Months, Fair Value	2,407	2,264
More than Twelve Months, Unrealized Losses	(93)	(236)
Total, Fair Value	56,556	7,658
Total, Unrealized Losses	(2,285)	(294)
Equity securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	1,144	969
Less than Twelve Months, Unrealized Losses	(41)	(75)
More than Twelve Months, Fair Value	12,601	12,664
More than Twelve Months, Unrealized Losses	(1,857)	(1,794)
Total, Fair Value	13,745	13,633
Total, Unrealized Losses	(1,898)	(1,869)
U.S. government agency securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	28,188
Less than Twelve Months, Unrealized Losses	(613)
More than Twelve Months, Fair Value
More than Twelve Months, Unrealized Losses
Total, Fair Value	28,188
Total, Unrealized Losses	(613)
Obligations of states and political subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	24,204
Less than Twelve Months, Unrealized Losses	(2,165)
More than Twelve Months, Fair Value
More than Twelve Months, Unrealized Losses
Total, Fair Value	24,204
Total, Unrealized Losses	$ (2,165)