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INVESTMENTS. (Investments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Investment [Line Items]
U.S. Treasury note	$ 1.0		$ 1.0		$ 1.0
Total	2,605.9		2,605.9		2,453.4
Net unrealized gains on T. Rowe Price investments product recognized in nonoperating income	4.8	$ 11.3	84.2	$ 17.4
Gains (losses) recognized upon deconsolidation	43.0	$ 124.9	370.3	$ 175.1
Daiwa SB Investments Ltd [Member]
Investment [Line Items]
Investments ownership percentage		10.00%		10.00%
Reclassification out of accumulated other comprehensive income [Member] \| Currency translation adjustments [Member]
Investment [Line Items]
Gains (losses) recognized upon deconsolidation	(0.1)	$ 0.0	0.1	$ 3.6
Assets [Member]
Investment [Line Items]
Net increase (decrease) in assets, liabilities, and redeemable noncontrolling interest	(113.5)	(41.1)	(169.6)	(701.5)
Liability [Member]
Investment [Line Items]
Net increase (decrease) in assets, liabilities, and redeemable noncontrolling interest	(9.8)	(0.6)	(11.0)	17.9
Redeemable noncontrolling interest [Member]
Investment [Line Items]
Net increase (decrease) in assets, liabilities, and redeemable noncontrolling interest	(72.3)	$ (19.8)	(81.4)	$ (595.0)
T. Rowe Price investment products [Member]
Investment [Line Items]
Investments held at fair value	1,568.0		1,568.0		1,538.4
Equity method investments	320.7		320.7		276.2
T. Rowe Price investment products designated as economic hedge of supplemental savings plan liability [Member]
Investment [Line Items]
Investments held at fair value	443.0		443.0		381.3
Investment partnerships and other investments [Member]
Investment [Line Items]
Investments held at fair value	103.2		103.2		99.6
Interest in UTI Asset Management Company Limited (India) [Member]
Investment [Line Items]
Equity method investments	$ 165.0		$ 165.0		$ 152.4
Equity method investment (ownership percentage)	26.00%		26.00%		26.00%
Investment partnerships and other investments [Member]
Investment [Line Items]
Equity method investments	$ 5.0		$ 5.0		$ 4.5