FAIR VALUE MEASUREMENTS. (Details) - USD ($) $ in Millions	Jun. 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	$ 1,705.1	$ 1,355.6
Other investments	0.2	0.0
Total	3,349.2	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	2.7	0.0
Total	12.6	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	90.5	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	992.4	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	992.4	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	132.2	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	122.3	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	9.9	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	529.2	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	529.2	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