Gross Unrealized Losses and Fair Value by Length of Time for Held-To-Maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Fair Value	$ 38,059	$ 541
Less than 12 months	417	1
More than 12 months
Total Unrealized Losses	417	1
Held-to-Maturity Securities
Gain (Loss) on Investments [Line Items]
Number of securities	23	1
US Treasury and Government
Gain (Loss) on Investments [Line Items]
Fair Value	17,435
Less than 12 months	140
More than 12 months
Total Unrealized Losses	140
US Treasury and Government | Held-to-Maturity Securities
Gain (Loss) on Investments [Line Items]
Number of securities	3
US States and Political Subdivisions Debt Securities
Gain (Loss) on Investments [Line Items]
Fair Value	20,624	541
Less than 12 months	277	1
More than 12 months
Total Unrealized Losses	$ 277	$ 1
US States and Political Subdivisions Debt Securities | Held-to-Maturity Securities
Gain (Loss) on Investments [Line Items]
Number of securities	20	1