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FAIR VALUE MEASUREMENTS. (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale investments |  | $ 597.1
Level 1 [Member] | Fair value, measurements, recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash equivalents | $ 1,488.9 | 1,726.4
T. Rowe Price investment products | 1,496.4 | 942.9
Total | 2,985.3 | 2,669.3
Level 2 [Member] | Fair value, measurements, recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash equivalents | 0.0 | 0.0
T. Rowe Price investment products | 18.0 | 17.4
Total | 18.0 | $ 17.4
Fair value measured at NAV [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
T. Rowe Price investment products | $ 95.2 |

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