FAIR VALUE MEASUREMENTS. (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Level 1 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents held in T. Rowe Price money market funds	$ 1,870.9	$ 1,355.6
Other investments	2.0	0.0
Total	3,593.5	3,309.7
Level 2 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents held in T. Rowe Price money market funds	0.0	0.0
Other investments	0.6	0.0
Total	10.9	9.9
Fair value measured at NAV
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products and other investments	96.8	99.7
T. Rowe Price investment products - discretionary investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products and other investments	1,005.6	1,221.8
T. Rowe Price investment products - discretionary investments | Level 1 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products and other investments	1,005.6	1,221.8
T. Rowe Price investment products - discretionary investments | Level 2 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products and other investments	0.0	0.0
T. Rowe Price investment products - seed capital
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products and other investments	160.0	181.1
T. Rowe Price investment products - seed capital | Level 1 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products and other investments	149.7	171.2
T. Rowe Price investment products - seed capital | Level 2 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products and other investments	10.3	9.9
T. Rowe Price investment products - supplemental savings plan liability economic hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products and other investments	565.3	561.1
T. Rowe Price investment products - supplemental savings plan liability economic hedges | Level 1 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products and other investments	565.3	561.1
T. Rowe Price investment products - supplemental savings plan liability economic hedges | Level 2 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products and other investments	$ 0.0	$ 0.0