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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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INVESTMENTS. (Investments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018
Investment [Line Items]
U.S. Treasury note	$ 1.0	$ 1.0
Total	2,453.4	1,477.3
Available for sale		597.1
Net unrealized gains on T. Rowe Price investments product recognized in nonoperating income	(18.5)
Gains recognized upon deconsolidation	23.2	396.3	$ 227.1
Currency translation adjustments [Member] \| Reclassification out of accumulated other comprehensive income [Member]
Investment [Line Items]
Gains recognized upon deconsolidation	3.6	0.1	2.2
Assets [Member]
Investment [Line Items]
Net increase (decrease) in assets, liabilities, and redeemable noncontrolling interest	736.6	1,397.2	1,342.8
Liability [Member]
Investment [Line Items]
Net increase (decrease) in assets, liabilities, and redeemable noncontrolling interest	17.6	(153.1)	(37.3)
Redeemable non-controlling interest [Member]
Investment [Line Items]
Net increase (decrease) in assets, liabilities, and redeemable noncontrolling interest	(626.4)	(1,042.6)	$ (967.3)
T. Rowe Price investment products [Member]
Investment [Line Items]
Equity method investments	276.2	277.4
T. Rowe Price investment products [Member]
Investment [Line Items]
Investment held at fair value	1,538.4	692.1
T. Rowe Price investment products designated as an economic hedge of supplemental savings plan liability [Member]
Investment [Line Items]
Investment held at fair value	381.3	268.2
Investment partnership and other investments [Member]
Investment [Line Items]
Investment held at fair value	99.6	78.0
Interest in UTI Asset Management Company Limited (India) [Member]
Investment [Line Items]
Equity method investments	$ 152.4	$ 155.8
Equity method investment (ownership percentage)	26.00%	26.00%
Investment partnerships [Member]
Investment [Line Items]
Equity method investments	$ 4.5	$ 4.8
Daiwa SB Investments Ltd [Member]
Investment [Line Items]
Ownership interest in investments (as percent)				10.00%