Accounting Policies - Additional Information (Detail)	12 Months Ended
Dec. 31, 2016
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.
Minimum [Member]
Accounting Policies [Line Items]
Short-term investments maturity period	3 months
Maximum [Member]
Accounting Policies [Line Items]
Cash equivalents maturity period	3 months
Short-term investments maturity period	12 months

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.

3 months