Fair Value Measurement - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available for sale securities, Fair value	$ 555,188	$ 574,562
Fair Value, Measurements, Recurring [Member] | US Treasury and government sponsored entities securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available for sale securities, Fair value	233,867	163,692
Fair Value, Measurements, Recurring [Member] | Mortgage-backed GSE securities: residential [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available for sale securities, Fair value	370	313
Fair Value, Measurements, Recurring [Member] | Mortgage-backed GSE securities: residential [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available for sale securities, Fair value	320,951	410,557
Fair Value, Measurements, Recurring [Member] | Interest Rate Cap [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available for sale securities, Fair value	877	436
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage-backed GSE securities: residential [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available for sale securities, Fair value	370	313
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | US Treasury and government sponsored entities securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available for sale securities, Fair value	233,867	163,692
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage-backed GSE securities: residential [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available for sale securities, Fair value	320,951	410,557
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Cap [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available for sale securities, Fair value	$ 877	$ 436