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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income attributable to APAM	$ 41,256	$ 42,518	$ 111,983	$ 125,798
Less: Allocation to participating securities	4,494	4,628	16,521	15,475
Net income available to common stockholders	$ 36,762	$ 37,890	$ 95,462	$ 110,323
Basic and diluted weighted average number of common share outstanding (in shares)	51,448,938	49,399,553	50,950,618	48,607,837
Earnings per share (in dollars per share)	$ 0.71	$ 0.77	$ 1.87	$ 2.27
Dilutive securities outstanding (in shares)	0	0	0	0

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The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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Amount of earnings (loss) distributed and earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Average number of shares or units issued and outstanding that are used in calculating basic and diluted earnings per share (EPS).

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