Annual report pursuant to Section 13 and 15(d)

Accounting Policies - Additional Information (Detail)

v3.8.0.1
Accounting Policies - Additional Information (Detail)
12 Months Ended
Dec. 31, 2017
segment
Accounting Policies [Line Items]  
Maturity period of liquid investments 3 months
Maturity period of financial instruments Greater than three months and less than one year
Number of operating segments 1
Restricted Stock Unit Awards [Member]  
Accounting Policies [Line Items]  
Award vesting period 4 years