Condensed Consolidated Statements of Cash Flows - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Cash flows from operating activities:
Net income	$ 34,000,000.0	$ 83,000,000.0	$ 110,000,000.0	$ 165,700,000
Less: Net income attributable to non-controlling interests in consolidated Funds	3,200,000	(7,600,000)	(21,300,000)	(9,600,000)
Adjustments to reconcile net income to net cash flows from operating activities from continuing operations:
Impairment of goodwill	0	0	16,400,000	0	$ 0
Amortization of acquired intangibles	1,600,000	1,600,000	5,100,000	4,900,000
(Gain) on sale of Affiliate	(7,200,000)		(7,200,000)
Changes in operating assets and liabilities (excluding discontinued operations):
Net cash flows from operating activities of continuing operations			92,000,000.0	(183,600,000)
Net cash flows from operating activities of discontinued operations			0	0
Total net cash flows from operating activities			92,000,000.0	(183,600,000)
Cash flows from investing activities:
Net cash flows from investing activities of continuing operations			(60,100,000)	(8,800,000)
Net cash flows from investing activities of discontinued operations			0	0
Total net cash flows from investing activities			(60,100,000)	(8,800,000)
Cash flows from financing activities:
Net cash flows from financing activities of continuing operations			(9,000,000.0)	(28,400,000)
Net cash flows from financing activities of discontinued operations			0	0
Total net cash flows from financing activities			(9,000,000.0)	(28,400,000)
Effect of foreign exchange rate changes on cash and cash equivalents			0	0
Net increase (decrease) in cash and cash equivalents, including cash classified within assets held for sale			22,900,000	(220,800,000)
Net increase (decrease) in cash and cash equivalents			9,900,000	(220,800,000)
Cash and cash equivalents at beginning of period			121,000,000.0	345,500,000	345,500,000
Cash and cash equivalents at end of period (including cash at consolidated Funds classified as restricted)	130,900,000	124,700,000	130,900,000	124,700,000	$ 121,000,000.0
Supplemental disclosure of cash flow information:
Interest paid (excluding consolidated Funds)			22,300,000	24,800,000
Income taxes paid			4,100,000	41,600,000
Consolidated Entity Excluding Consolidated Funds
Adjustments to reconcile net income to net cash flows from operating activities from continuing operations:
Impairment of goodwill			16,400,000	0
Amortization of acquired intangibles	1,600,000	1,600,000	5,100,000	4,900,000
(Gain) on sale of Affiliate	(7,200,000)	0	(7,200,000)	0
Depreciation and other amortization	5,200,000	4,400,000	15,600,000	12,200,000
Amortization of debt-related costs			3,200,000	2,600,000
Amortization and revaluation of non-cash compensation awards			(17,400,000)	6,300,000
Net earnings from Affiliate accounted for using the equity method			(2,200,000)	(2,100,000)
Distributions received from equity method Affiliate			2,200,000	2,300,000
Deferred income taxes			30,100,000	7,600,000
(Gains) losses on other investments			1,700,000	(26,300,000)
Changes in operating assets and liabilities (excluding discontinued operations):
Decrease in investment advisory fees receivable			26,200,000	12,200,000
Increase in other receivables, prepayments, deposits and other assets			(2,600,000)	(8,800,000)
Decrease in accrued incentive compensation, operating lease liabilities and other liabilities			(5,900,000)	(262,800,000)
Decrease in accounts payable, accrued expenses and accrued income taxes			(9,200,000)	(54,100,000)
Net cash flows from operating activities of continuing operations			144,700,000	(149,900,000)
Purchase of investments			(13,900,000)	(18,000,000.0)
Earnings from equity method investees			(2,200,000)	(2,100,000)
Cash flows from investing activities:
Additions of fixed assets			(22,500,000)	(26,700,000)
Sale of investments			62,900,000	40,100,000
Cash flows from financing activities:
Proceeds from third party and non-recourse borrowings			80,000,000.0	485,000,000.0
Repayment of third party and non-recourse borrowings			(175,000,000.0)	(275,000,000.0)
Payment for debt issuance costs			0	(1,800,000)
Payment to OM plc for DTA Deed			0	(32,700,000)
Payment to OM plc for co-investment redemptions			(300,000)	(5,100,000)
Dividends paid to stockholders			(6,700,000)	(18,800,000)
Dividends paid to related parties			(3,300,000)	(8,600,000)
Repurchases of common stock			(35,300,000)	(212,600,000)
Less: net (increase) in cash classified within Affiliate and Consolidated funds' assets held for sale			(12,200,000)	0
Consolidated Funds
Cash flows from operating activities:
Less: Net income attributable to non-controlling interests in consolidated Funds	$ 3,200,000	$ (7,600,000)	(21,300,000)	(9,600,000)
Adjustments to reconcile net income to net cash flows from operating activities from continuing operations:
Net earnings from Affiliate accounted for using the equity method			(28,100,000)	(11,200,000)
(Gains) losses on other investments			11,300,000	6,100,000
Changes in operating assets and liabilities (excluding discontinued operations):
Net cash flows from operating activities of continuing operations			(52,700,000)	(33,700,000)
Purchase of investments			(98,200,000)	(161,100,000)
Sale of investments			66,700,000	122,100,000
Earnings from equity method investees			(28,100,000)	(11,200,000)
(Increase) decrease in receivables and other assets			(28,700,000)	4,800,000
Increase (decrease) in accounts payable and other liabilities			3,000,000.0	(4,000,000.0)
Cash flows from investing activities:
Contributions in equity method investees			(1,900,000)	(8,000,000.0)
Distributions received from equity method investees			1,300,000	3,800,000
Deconsolidation of Funds			(86,000,000.0)	0
Cash flows from financing activities:
Non-controlling interest capital raised			1,900,000	8,000,000.0
Non-controlling interest capital redeemed			(400,000)	(3,100,000)
Redeemable non-controlling interest capital raised			131,200,000	38,000,000.0
Redeemable non-controlling interest capital redeemed			(1,100,000)	(1,700,000)
Less: net (increase) in cash classified within Affiliate and Consolidated funds' assets held for sale			(800,000)	0
Supplemental disclosure of non-cash investing and financing transactions:
Payable for securities purchased by a consolidated Fund			$ 0	$ 7,400,000