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INFORMATION ABOUT RECEIVABLES, REVENUES, AND SERVICES. (Investment Advisory Services) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Revenue and Services [Line Items]      
Revenues from advisory services $ 4,287.7 $ 3,728.7 $ 3,687.3
Average assets under management 909,000.0 778,200.0 767,900.0
Period end assets under management 991,100.0 810,800.0  
T. Rowe Price U.S. mutual funds [Member]      
Revenue and Services [Line Items]      
Revenues from advisory services 3,071.9 2,705.4 2,667.9
Average assets under management 568,500.0 495,500.0 493,600.0
Period end assets under management 606,300.0 514,200.0  
T. Rowe Price U.S. mutual funds [Member] | Stock and blended asset [Member]      
Revenue and Services [Line Items]      
Revenues from advisory services 2,570.9 2,228.1 2,241.9
Average assets under management 447,500.0 386,100.0 387,800.0
Period end assets under management 480,500.0 401,300.0  
T. Rowe Price U.S. mutual funds [Member] | Bond and money market [Member]      
Revenue and Services [Line Items]      
Revenues from advisory services 501.0 477.3 426.0
Average assets under management 121,000.0 109,400.0 105,800.0
Period end assets under management 125,800.0 112,900.0  
Other investment products [Member]      
Revenue and Services [Line Items]      
Revenues from advisory services 1,215.8 1,023.3 1,019.4
Average assets under management 340,500.0 282,700.0 274,300.0
Period end assets under management 384,800.0 296,600.0  
Other investment products [Member] | Stock and blended asset [Member]      
Revenue and Services [Line Items]      
Revenues from advisory services 1,009.4 850.3 862.2
Average assets under management 256,400.0 211,100.0 210,300.0
Period end assets under management 291,900.0 220,800.0  
Other investment products [Member] | Bond, money market, and stable value [Member]      
Revenue and Services [Line Items]      
Revenues from advisory services 206.4 173.0 157.2
Average assets under management 84,100.0 71,600.0 64,000.0
Period end assets under management 92,900.0 75,800.0  
Other T. Rowe Price investment products [Member]      
Revenue and Services [Line Items]      
Revenues from advisory services $ 478.2 386.0 367.8
Waived money market related fees [Member]      
Revenue and Services [Line Items]      
Revenues from advisory services   $ 10.5 $ 47.6