Investment Securities - Amount of securities in unrealized loss position (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 220,604	$ 62,697
Less than Twelve Months, Unrealized Losses	(5,383)	(300)
Twelve Months or Longer, Fair value		480
Twelve Months or Longer, Unrealized Losses		(12)
Total, Fair Value	220,604	63,177
Total gross, Unrealized Losses	(5,383)	(312)
U.S. treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	4,763	4,938
Less than Twelve Months, Unrealized Losses	(200)	(22)
Twelve Months or Longer, Fair value
Twelve Months or Longer, Unrealized Losses
Total, Fair Value	4,763	4,938
Total gross, Unrealized Losses	(200)	(22)
U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	111,718	36,793
Less than Twelve Months, Unrealized Losses	(2,257)	(75)
Twelve Months or Longer, Fair value
Twelve Months or Longer, Unrealized Losses
Total, Fair Value	111,718	36,793
Total gross, Unrealized Losses	(2,257)	(75)
State and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	25,144	4,574
Less than Twelve Months, Unrealized Losses	(911)	(14)
Twelve Months or Longer, Fair value		480
Twelve Months or Longer, Unrealized Losses		(12)
Total, Fair Value	25,144	5,054
Total gross, Unrealized Losses	(911)	(26)
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	25,535
Less than Twelve Months, Unrealized Losses	(148)
Twelve Months or Longer, Fair value
Twelve Months or Longer, Unrealized Losses
Total, Fair Value	25,535
Total gross, Unrealized Losses	(148)
Collateralized mortgage obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	6,602	5,006
Less than Twelve Months, Unrealized Losses	(93)	(5)
Twelve Months or Longer, Fair value
Twelve Months or Longer, Unrealized Losses
Total, Fair Value	6,602	5,006
Total gross, Unrealized Losses	(93)	(5)
Corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	45,778	10,410
Less than Twelve Months, Unrealized Losses	(1,758)	(51)
Twelve Months or Longer, Fair value
Twelve Months or Longer, Unrealized Losses
Total, Fair Value	45,778	10,410
Total gross, Unrealized Losses	(1,758)	(51)
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	1,064	976
Less than Twelve Months, Unrealized Losses	(16)	(133)
Twelve Months or Longer, Fair value
Twelve Months or Longer, Unrealized Losses
Total, Fair Value	1,064	976
Total gross, Unrealized Losses	$ (16)	$ (133)