FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Millions	Sep. 30, 2015	Jul. 01, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Assets
Derivative assets	$ 0.3
Total investment securities	357.3			$ 344.0
Liabilities
Redeemable noncontrolling interests	36.3			5.4
Kapstream
Liabilities
contingent consideration	5.6	$ 6.0
Futures
Assets
Derivative assets	0.2
Index Swap
Assets
Derivative assets	0.1			0.3
Seeded Investment Products
Assets
Derivative assets	1.6			1.4
Liabilities
Derivative liabilities	0.2
Redeemable noncontrolling interests	28.8		$ 21.4	41.1	$ 38.8	$ 14.4	$ 8.8
Seeded Investment Products | Futures
Assets
Derivative assets				0.2
Liabilities
Derivative liabilities	0.1
Seeded Investment Products | Index Swap
Assets
Derivative assets	1.6			1.0
Liabilities
Derivative liabilities	$ 0.1
Trading Securities | Seeded Investment Products | Minimum
Assets and liabilities carried at fair value
Minimum controlling interest required for consolidation (as a percent)	50.00%
Trading Securities | Seeded Investment Products | Maximum
Assets and liabilities carried at fair value
Ownership percentage to classify security as level 1	50.00%
INTECH founding member ownership interests
Liabilities
Redeemable noncontrolling interests	$ 5.6			5.9
Fair Value, Measurements, Recurring
Assets
Cash equivalents	167.2			300.2
Total investment securities	357.3			344.0
Total assets	525.0			644.5
Liabilities
Current portion of long-term debt	154.5
Long-term debt	306.6			558.0
Total liabilities	488.9			577.1
Redeemable noncontrolling interests	36.3
Fair Value, Measurements, Recurring | Kapstream
Liabilities
contingent consideration	5.6
Fair Value, Measurements, Recurring | VelocityShares
Liabilities
contingent consideration	21.8			17.9
Fair Value, Measurements, Recurring | Futures
Assets
Derivative assets	0.4
Liabilities
Derivative liabilities	0.2			1.2
Fair Value, Measurements, Recurring | Index Swap
Assets
Derivative assets	0.1			0.3
Fair Value, Measurements, Recurring | Credit Default Swap
Liabilities
Derivative liabilities	0.2
Fair Value, Measurements, Recurring | Seeded Investment Products
Assets
Trading securities	267.3			258.3
Available-for-sale securities, Estimated fair value	70.2			68.3
Liabilities
Redeemable noncontrolling interests	28.8
Fair Value, Measurements, Recurring | Investments in advised funds
Assets
Trading securities	4.0			4.4
Fair Value, Measurements, Recurring | Investments related to deferred compensation plans
Assets
Trading securities	15.8			13.0
Fair Value, Measurements, Recurring | INTECH founding member ownership interests
Liabilities
Redeemable noncontrolling interests	7.5			5.4
Fair Value, Measurements, Recurring | Level 1
Assets
Cash equivalents	46.3			7.7
Total investment securities	262.5			222.3
Total assets	309.3			230.0
Liabilities
Total liabilities	0.4			1.2
Redeemable noncontrolling interests	5.8
Fair Value, Measurements, Recurring | Level 1 | Futures
Assets
Derivative assets	0.4
Liabilities
Derivative liabilities	0.2			1.2
Fair Value, Measurements, Recurring | Level 1 | Index Swap
Assets
Derivative assets	0.1
Fair Value, Measurements, Recurring | Level 1 | Credit Default Swap
Liabilities
Derivative liabilities	0.2
Fair Value, Measurements, Recurring | Level 1 | Seeded Investment Products
Assets
Trading securities	172.5			136.6
Available-for-sale securities, Estimated fair value	70.2			68.3
Liabilities
Redeemable noncontrolling interests	5.8
Fair Value, Measurements, Recurring | Level 1 | Investments in advised funds
Assets
Trading securities	4.0			4.4
Fair Value, Measurements, Recurring | Level 1 | Investments related to deferred compensation plans
Assets
Trading securities	15.8			13.0
Fair Value, Measurements, Recurring | Level 2
Assets
Cash equivalents	120.9			292.5
Total investment securities	94.8			121.7
Total assets	215.7			414.5
Liabilities
Current portion of long-term debt	154.5
Long-term debt	306.6			558.0
Total liabilities	461.1			558.0
Redeemable noncontrolling interests	23.0
Fair Value, Measurements, Recurring | Level 2 | Index Swap
Assets
Derivative assets				0.3
Fair Value, Measurements, Recurring | Level 2 | Seeded Investment Products
Assets
Trading securities	94.8			121.7
Liabilities
Redeemable noncontrolling interests	23.0
Fair Value, Measurements, Recurring | Level 3
Liabilities
Total liabilities	27.4			17.9
Redeemable noncontrolling interests	7.5
Fair Value, Measurements, Recurring | Level 3 | Kapstream
Liabilities
contingent consideration	5.6
Fair Value, Measurements, Recurring | Level 3 | VelocityShares
Liabilities
contingent consideration	21.8			17.9
Fair Value, Measurements, Recurring | Level 3 | INTECH founding member ownership interests
Liabilities
Redeemable noncontrolling interests	$ 7.5			$ 5.4