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Short-term Investments - Additional Information (Detail) - US Treasury Securities and Certificates of Deposit - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Investment [Line Items]    
Held to maturity securities, amortized cost $ 259,800,000 $ 187,300,000
Held to maturity securities, carrying value 259,800,000 187,300,000
Held to maturity securities, fair value 259,700,000 187,300,000
Other-than-temporary impairments related to these investments $ 0 $ 0
Minimum    
Investment [Line Items]    
Held to maturity securities maturity range 3 months  
Maximum    
Investment [Line Items]    
Held to maturity securities maturity range 1 year