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Fair Value Measurements - Changes in Level 3 Assets Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward    
Balance at the beginning of the year $ 21,385 $ 21,124
Purchases 1,668 1,096
Settlements (722) (191)
Sales (5,412) (429)
(Losses) gains included in earnings 4,060 (111)
Total unrealized losses (808) (104)
Balance at the end of the period $ 20,171 $ 21,385