Fair Value Measurements and Disclosures - Assets Measured at Fair Value on Non-Recurring Basis of Impairment Loans (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Dec. 31, 2011
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value measured on non-recurring basis, impaired loans	$ 26,791	$ 16,030
Level 1 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value measured on non-recurring basis, impaired loans
Level 2 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value measured on non-recurring basis, impaired loans
Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value measured on non-recurring basis, impaired loans	$ 26,791	$ 16,030	$ 14,770