Quarterly report pursuant to Section 13 or 15(d)

Intangible Assets, Net - Summary of Intangible Assets (Detail)

v3.10.0.1
Intangible Assets, Net - Summary of Intangible Assets (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Cost of intangible assets gross $ 25,027   $ 25,027   $ 25,027 $ 25,027
Accumulated amortization, beginning balance     (13,449) $ (11,623) (11,623)  
Amortization expense (420) $ (456) (1,265) (1,369) (1,826)  
Accumulated amortization, ending balance (14,714)   (14,714)   (13,449)  
Net book value 10,313   $ 10,313   11,578  
Minimum            
Useful lives     3 years      
Maximum            
Useful lives     20 years      
Customer Relationships            
Cost of intangible assets gross 22,560   $ 22,560   22,560 22,560
Accumulated amortization, beginning balance     (11,367) (9,721) (9,721)  
Amortization expense     (1,140)   (1,646)  
Accumulated amortization, ending balance (12,507)   (12,507)   (11,367)  
Net book value 10,053   $ 10,053   $ 11,193  
Customer Relationships | Minimum            
Useful lives     10 years   10 years  
Customer Relationships | Maximum            
Useful lives     20 years   20 years  
Other Intangible Assets            
Cost of intangible assets gross 2,467   $ 2,467   $ 2,467 $ 2,467
Accumulated amortization, beginning balance     (2,082) $ (1,902) (1,902)  
Amortization expense     (125)   (180)  
Accumulated amortization, ending balance (2,207)   (2,207)   (2,082)  
Net book value $ 260   $ 260   $ 385  
Other Intangible Assets | Minimum            
Useful lives     3 years   3 years  
Other Intangible Assets | Maximum            
Useful lives     5 years   5 years