XML 38 R28.htm IDEA: XBRL DOCUMENT v3.5.0.2
Short-term Investments - Additional Information (Detail) - US Treasury Securities - USD ($)
9 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Investment [Line Items]    
Held to maturity securities, amortized cost $ 213,300,000 $ 193,300,000
Held to maturity securities, carrying value 213,300,000 193,300,000
Held to maturity securities, fair value 213,300,000 193,200,000
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