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Related Party Transactions - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Related Party Transaction                      
Expenses from transactions with related party $ 10,529 $ 8,740 $ 9,370 $ 9,534 $ 10,294 $ 10,105 $ 10,102 $ 10,221 $ 38,173 $ 40,722 $ 37,556
Investment in money market or mutual funds managed by affiliates of a related party 172,507       135,617       $ 172,507 135,617  
Management                      
Related Party Transaction                      
Fee paid by company to manager as percentage of company's shareholders' equity                 1.50%    
Termination fee multiplier                 3    
Termination fees assessment period                 24 months    
Invesco Advisers, Inc. | Affiliated Entity                      
Related Party Transaction                      
Expenses from transactions with related party                 $ 888 779 $ 801
Investment in money market or mutual funds managed by affiliates of a related party $ 154,000       $ 131,900       $ 154,000 $ 131,900