FAIR VALUE MEASUREMENTS. (Details) - USD ($) $ in Millions	Mar. 31, 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	$ 1,164.9	$ 1,355.6
Total	2,740.6	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
Total	12.4	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	105.5	99.7
T. Rowe Price investment products
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products	1,088.1	1,402.9
T. Rowe Price investment products | Level 1 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products	1,075.7	1,393.0
T. Rowe Price investment products | Level 2 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products	12.4	9.9
T. Rowe Price investment products designated as an economic hedge of supplemental savings plan liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products	500.0	561.1
T. Rowe Price investment products designated as an economic hedge of supplemental savings plan liability | Level 1 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products	500.0	561.1
T. Rowe Price investment products designated as an economic hedge of supplemental savings plan liability | Level 2 | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
T. Rowe Price investment products	$ 0.0	$ 0.0