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Fair Value Measurements - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value Disclosures [Abstract]      
Transfers of assets from level 1 to 2 $ 0 $ 0 $ 0
Transfers of assets from level 2 to 1 0 0 0
Transfers of liabilities from level 1 to 2 0 0 0
Transfers of liabilities from level 2 to 1 0 0 0
Transfers of assets into level 3 0 0 0
Transfers of assets out of level 3 0 0 0
Transfers of liabilities into level 3 0 0 0
Transfers of liabilities out of level 3 $ 0 $ 0 $ 0