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Fair Value Measurements - Change in Estimated Fair Values Calculated for Investment Valued on Recurring Basis Utilizing Level 3 Inputs (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Fair Value Disclosures [Abstract]  
Balance at the beginning of the period $ 2,508
Credit gain on available-for-sale security included in Other income (expense), net 11
Gain included in Other comprehensive income (loss) 6
Balance at the end of the period $ 2,525