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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, 2018
Dec. 31, 2017
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward    
Balance at the beginning of the year $ 20,171 $ 21,385
Purchases 3,190 1,668
Settlements (2,446) (722)
Sales (75) (5,412)
Gains included in earnings 118 4,060
Total unrealized gains/(losses) 6 (808)
Balance at the end of the period $ 20,964 $ 20,171