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FAIR VALUE MEASUREMENTS. (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Level 1 [Member] | Fair value, measurements, recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents held in T. Rowe Price money market funds $ 1,398.3 $ 1,196.0
T. Rowe Price investment products 1,964.1 1,900.5
Total 3,362.4 3,096.5
Level 2 [Member] | Fair value, measurements, recurring [Member]    
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
T. Rowe Price investment products 7.5 19.2
Total 7.5 19.2
Fair value measured at NAV [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
T. Rowe Price investment products $ 99.1 $ 99.6