Fair Value Measurements and Disclosures - Assets Measured at Fair Value on Non-Recurring Basis of Other Real Estate (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]
Assets measured at fair value on a non-recurring basis, other real estate	$ 6,825	$ 7,008
Level 1 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Assets measured at fair value on a non-recurring basis, other real estate
Level 2 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Assets measured at fair value on a non-recurring basis, other real estate
Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Assets measured at fair value on a non-recurring basis, other real estate	$ 6,825	$ 7,008	$ 6,153