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Earnings Per Share - Computation of basic and diluted net income per share (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Earnings Per Share [Abstract]        
Net income attributable to APAM $ 58,527 $ 41,256 $ 139,503 $ 111,983
Less: Allocation to participating securities 6,006 4,494 16,944 16,521
Net income available to common stockholders $ 52,521 $ 36,762 $ 122,559 $ 95,462
Basic weighted average number of common shares outstanding 56,402,503 51,448,938 55,188,563 50,950,618
Weighted Average Number Diluted Shares Outstanding Adjustment 5,769 0 1,923 0
Diluted weighted average number of common shares outstanding 56,408,272 51,448,938 55,190,486 50,950,618
Earnings per share - Basic (in dollars per share) $ 0.93 $ 0.71 $ 2.22 $ 1.87
Earnings per share - Diluted (in dollars per share) $ 0.93 $ 0.71 $ 2.22 $ 1.87