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Financial Instruments - Summary of Bond Activity (Detail) (USD $)
In Thousands, unless otherwise specified
6 Months Ended 12 Months Ended
Jun. 30, 2013
Dec. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]    
Fair value at beginning of period $ 29,776 $ 30,923
Purchases 4,390 5,742
Sales (345) (2,000)
Maturities and calls (3,000) (4,900)
Decrease in bond premium (154) (290)
Fair market value adjustment (295) 301
Fair value at end of period $ 30,372 $ 29,776