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Fair Value Measurements - Changes in Level 3 Assets Measured at Estimated Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation [Roll Forward]    
Balance at the beginning of the year $ 10,779 $ 13,225
Purchases, issuances and settlements 239 1,739
Sales (121) (5,154)
(Losses) gains included in earnings 5 (4,110)
Total unrealized (losses) gains (437) (2,518)
Balance at the end of the period $ 10,465 $ 11,402