Gross Unrealized Losses and Fair Value by Length of Time for Held-To-Maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Fair Value	$ 9,913	$ 541
Less than 12 months	60	1
More than 12 months
Total Unrealized Losses	60	1
Held-to-Maturity Securities
Gain (Loss) on Investments [Line Items]
Number of securities	9	1
US States and Political Subdivisions Debt Securities
Gain (Loss) on Investments [Line Items]
Fair Value	9,913	541
Less than 12 months	60	1
More than 12 months
Total Unrealized Losses	$ 60	$ 1
US States and Political Subdivisions Debt Securities | Held-to-Maturity Securities
Gain (Loss) on Investments [Line Items]
Number of securities	9	1