Annual report pursuant to Section 13 and 15(d)

Commitments and Contingencies - Additional Information (Detail)

v3.8.0.1
Commitments and Contingencies - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Amendment Two [Member]      
Operating Leased Assets [Line Items]      
Lease amendment commenced year 2017    
Lease expiration period 2027    
Maximum [Member]      
Operating Leased Assets [Line Items]      
Rent expense $ 1,000,000 $ 1,000,000 $ 1,000,000
Future gross minimum lease payments $ 1,000,000    
Lease One [Member]      
Operating Leased Assets [Line Items]      
Lease commenced year 2011    
Lease One [Member] | Amendment One [Member]      
Operating Leased Assets [Line Items]      
Lease amended year 2013    
Lease One [Member] | Amendment Two [Member]      
Operating Leased Assets [Line Items]      
Lease amended year 2017    
Lease Two [Member]      
Operating Leased Assets [Line Items]      
Lease commenced year 2012    
Lease Two [Member] | Amendment One [Member]      
Operating Leased Assets [Line Items]      
Lease amendment commenced year 2014