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Commitments and Contingencies - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 20, 2017
ft²
Option
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2020
ft²
Commitments And Contingencies [Line Items]          
Rent expense charged to operations   $ 5.6 $ 4.9 $ 5.0  
Current Headquarters [Member]          
Commitments And Contingencies [Line Items]          
Lease agreement expires, date Apr. 30, 2025        
Number of options to extend term of the lease | Option 2        
Lease extension period, number of years 5 years        
Incentives received upon execution of amendment   3.2      
Incentives Owed Upon Expansion of Square Footage   $ 2.1      
Current Headquarters [Member] | Minneapolis, Minnesota [Member]          
Commitments And Contingencies [Line Items]          
Current leased space for headquarters | ft² 189,000        
Current Headquarters [Member] | Scenario, Forecast [Member] | Minneapolis, Minnesota [Member]          
Commitments And Contingencies [Line Items]          
Future expansion of headquarters premises | ft²         25,000