OTHER COMPREHENSIVE INCOME AND ACCUMULATED OTHER COMPREHENSIVE INCOME. (Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 6,311.0	$ 5,901.9	$ 6,124.3	$ 5,824.4
Other comprehensive income (loss)	3.1	(17.3)	3.5	(9.9)
Ending Balance	6,579.1	6,114.6	6,579.1	6,114.6
Currency translation adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(41.6)	(6.4)	(42.0)	(13.8)
Other comprehensive income (loss) before reclassifications and income taxes	4.2	(22.8)	4.6	(9.8)
Reclassification adjustments recognized in non-operating income	(0.1)	(0.5)	(0.2)	(3.6)
Other comprehensive income (loss), before taxes	4.1	(23.3)	4.4	(13.4)
Net deferred tax benefits (income taxes)	(1.0)	6.0	(0.9)	3.5
Other comprehensive income (loss)	3.1	(17.3)	3.5	(9.9)
Ending Balance	(38.5)	(23.7)	(38.5)	(23.7)
Currency translation adjustments [Member] \| Equity method investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(45.0)	(34.0)	(48.8)	(37.0)
Other comprehensive income (loss) before reclassifications and income taxes	1.5	(3.3)	6.4	0.5
Reclassification adjustments recognized in non-operating income	0.0	0.0	0.0	0.0
Other comprehensive income (loss), before taxes	1.5	(3.3)	6.4	0.5
Net deferred tax benefits (income taxes)	(0.3)	0.7	(1.4)	(0.1)
Other comprehensive income (loss)	1.2	(2.6)	5.0	0.4
Ending Balance	(43.8)	(36.6)	(43.8)	(36.6)
Currency translation adjustments [Member] \| Consolidated T. Rowe Price investment products - variable interest entities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	3.4	27.6	6.8	23.2
Other comprehensive income (loss) before reclassifications and income taxes	2.7	(19.5)	(1.8)	(10.3)
Reclassification adjustments recognized in non-operating income	(0.1)	(0.5)	(0.2)	(3.6)
Other comprehensive income (loss), before taxes	2.6	(20.0)	(2.0)	(13.9)
Net deferred tax benefits (income taxes)	(0.7)	5.3	0.5	3.6
Other comprehensive income (loss)	1.9	(14.7)	(1.5)	(10.3)
Ending Balance	$ 5.3	$ 12.9	$ 5.3	$ 12.9

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