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Fair Value Measurement - Additional Information (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]    
Fair value, assets, level 1 to level 2 transfers, amount $ 0 $ 0
Fair value, assets, level 2 to level 1 transfers, amount 0 0
Fair value, liabilities, level 1 to level 2 transfers, amount 0 0
Fair value, liabilities, level 2 to level 1 transfers, amount 0 0
Fair value, equity, level 1 to level 2 transfers, amount 0 0
Fair value, equity, level 2 to level 1 transfers, amount $ 0 $ 0