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Accounting Policies - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2017
Accounting Policies [Line Items]  
Income tax, Uncertain tax position description The tax benefits recognized in the financial statements from such positions are then measured based on the largest benefit that has a greater than 50 percent likelihood of being realized upon settlement.
Maximum [Member]  
Accounting Policies [Line Items]  
Cash equivalents maturity period 3 months