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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Earnings Per Share [Abstract]
Net income attributable to APAM	$ 44,279	$ 88,207	$ 109,712	$ 165,485
Less: Allocation to participating securities	5,903	8,765	15,265	15,120
Net income available to common stockholders	$ 38,376	$ 79,442	$ 94,447	$ 150,365
Basic weighted average shares outstanding (in shares)	62,320,372	59,821,804	62,180,483	59,292,982
Dilutive effect of nonparticipating share-based awards (in shares)	30,011	16,570	30,665	15,777
Diluted weighted average shares outstanding (in shares)	62,350,383	59,838,374	62,211,148	59,308,759
Earnings per share - Basic (in dollars per share)	$ 0.62	$ 1.33	$ 1.52	$ 2.54
Earnings per share - Diluted (in dollars per share)	$ 0.62	$ 1.33	$ 1.52	$ 2.54