EARNINGS PER SHARE CALCULATIONS. (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Stock options with anti-diluted impact (in shares)	0	0	0	0
Net income attributable to T. Rowe Price Group	$ 643.2	$ 545.9	$ 1,589.3	$ 1,586.0
Less: net income allocated to outstanding restricted stock and stock unit holders (basic)	17.9	13.9	44.1	40.7
Less: net income allocated to outstanding restricted stock and stock unit holders (diluted)	17.9	13.9	44.1	40.7
Net income allocated to common stockholders (basic)	625.3	532.0	1,545.2	1,545.3
Net income allocated to common stockholders (diluted)	$ 625.3	$ 532.0	$ 1,545.2	$ 1,545.3
Weighted-average common shares
Outstanding (in shares)	227,000,000.0	234,900,000	229,500,000	235,800,000
Outstanding assuming dilution (in shares)	229,400,000	238,200,000	231,900,000	239,000,000.0