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Assets and Liabilities Measured at Fair Value (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Collateral for Redeemable Convertible Notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted cash $ 5.2 $ 19.4
Collateral for Corporate Credit Card and Deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted cash $ 0.1 $ 0.1