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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 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Earnings Per Share [Abstract]        
Net income attributable to APAM $ 88,207 $ 46,151 $ 165,485 $ 80,976
Less: Allocation to participating securities 8,765 6,001 15,120 12,155
Net income available to common stockholders $ 79,442 $ 40,150 $ 150,365 $ 68,821
Basic weighted average shares outstanding (in shares) 59,821,804 55,884,366 59,292,982 54,574,923
Dilutive effect of nonparticipating share-based awards (in shares) 16,570 0 15,777 0
Diluted weighted average shares outstanding 59,838,374 55,884,366 59,308,759 54,574,923
Earnings per share - Basic (in dollars per share) $ 1.33 $ 0.72 $ 2.54 $ 1.26
Earnings per share - Diluted (in dollars per share) $ 1.33 $ 0.72 $ 2.54 $ 1.26