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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income attributable to APAM	$ 42,006	$ 26,632	$ 83,280	$ 46,427
Less: Allocation to participating securities	6,852	6,339	11,775	9,198
Net income available to common stockholders	$ 35,154	$ 20,293	$ 71,505	$ 37,229
Weighted average shares outstanding (in shares)	49,041,113	45,241,102	48,205,418	43,142,011
Earnings per share (in dollars per share)	$ 0.72	$ 0.45	$ 1.48	$ 0.86
Dilutive securities outstanding (in shares)	0	0	0	0