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Earnings Per Share and Dividends - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Oct. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Dividends Payable [Line Items]                  
Cash dividends declared per share of Class A common stock               $ 0.60 $ 0.54
Dividends declared and paid               $ 10,645 $ 9,554
Distributions declared to non-controlling unitholders               $ 7,536 $ 6,783
Common Class A                  
Dividends Payable [Line Items]                  
Cash dividends declared per share of Class A common stock   $ 0.20 $ 0.20 $ 0.20 $ 0.18 $ 0.18 $ 0.18 $ 0.60 $ 0.54
Dividends declared and paid   $ 3,549 $ 3,549 $ 3,547 $ 3,185 $ 3,185 $ 3,184    
Quarterly dividend | Common Class A                  
Dividends Payable [Line Items]                  
Cash dividends declared per share of Class A common stock $ 0.20                
Non-controlling interest                  
Dividends Payable [Line Items]                  
Distributions declared to non-controlling unitholders   $ 2,512 $ 2,512 $ 2,512 $ 2,261 $ 2,261 $ 2,261