Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	$ 496,155	$ 374,643
Less than Twelve Months, Unrealized Losses	(8,154)	(4,118)
More than Twelve Months, Fair Value	68,612	63,882
More than Twelve Months, Unrealized Losses	(11,204)	(15,361)
Total, Fair Value	564,767	438,525
Total, Unrealized Losses	(19,358)	(19,479)
U.S. Treasury securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	95,458	97,625
Less than Twelve Months, Unrealized Losses	(4,383)	(2,218)
More than Twelve Months, Fair Value
More than Twelve Months, Unrealized Losses
Total, Fair Value	95,458	97,625
Total, Unrealized Losses	(4,383)	(2,218)
Residential mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	321,565	269,895
Less than Twelve Months, Unrealized Losses	(2,988)	(1,256)
More than Twelve Months, Fair Value	20,963	21,089
More than Twelve Months, Unrealized Losses	(2,020)	(2,103)
Total, Fair Value	342,528	290,984
Total, Unrealized Losses	(5,008)	(3,359)
Trust preferred securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value		760
Less than Twelve Months, Unrealized Losses		(511)
More than Twelve Months, Fair Value	32,458	27,865
More than Twelve Months, Unrealized Losses	(7,416)	(11,228)
Total, Fair Value	32,458	28,625
Total, Unrealized Losses	(7,416)	(11,739)
Corporate and other debt securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	34,240	5,394
Less than Twelve Months, Unrealized Losses	(135)	(58)
More than Twelve Months, Fair Value	2,404	2,264
More than Twelve Months, Unrealized Losses	(96)	(236)
Total, Fair Value	36,644	7,658
Total, Unrealized Losses	(231)	(294)
Equity securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	947	969
Less than Twelve Months, Unrealized Losses	(9)	(75)
More than Twelve Months, Fair Value	12,787	12,664
More than Twelve Months, Unrealized Losses	(1,672)	(1,794)
Total, Fair Value	13,734	13,633
Total, Unrealized Losses	(1,681)	(1,869)
U.S. government agency securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	17,734
Less than Twelve Months, Unrealized Losses	(7)
More than Twelve Months, Fair Value
More than Twelve Months, Unrealized Losses
Total, Fair Value	17,734
Total, Unrealized Losses	(7)
Obligations of states and political subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than Twelve Months, Fair Value	26,211
Less than Twelve Months, Unrealized Losses	(632)
More than Twelve Months, Fair Value
More than Twelve Months, Unrealized Losses
Total, Fair Value	26,211
Total, Unrealized Losses	$ (632)