Borrowings and Debt - Long Term Debt Excluding Consolidated Funds (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Face amount	$ 350,000,000
Senior notes \| $275 million 4.80% Senior Notes Due July 27, 2026
Debt Instrument [Line Items]
Face amount	$ 275,000,000
Interest rate	4.80%
Senior notes \| $125 million 5.125% Senior Notes Due August 1, 2031
Debt Instrument [Line Items]
Face amount	$ 125,000,000
Interest rate	5.125%
Fair Value, Inputs, Level 2
Debt Instrument [Line Items]
Carrying value	$ 628,500,000	$ 393,300,000
Fair Value	636,300,000	368,300,000
Fair Value, Inputs, Level 2 \| Line of Credit \| Non-recourse seed capital facility expiring July 17, 2020 (2)
Debt Instrument [Line Items]
Carrying value	25,000,000.0	0
Fair Value	25,000,000.0	0
Fair Value, Inputs, Level 2 \| Senior notes \| $275 million 4.80% Senior Notes Due July 27, 2026
Debt Instrument [Line Items]
Carrying value	272,300,000	272,200,000
Fair Value	278,300,000	266,000,000.0
Fair Value, Inputs, Level 2 \| Senior notes \| $125 million 5.125% Senior Notes Due August 1, 2031
Debt Instrument [Line Items]
Carrying value	121,200,000	121,100,000
Fair Value	123,000,000.0	102,300,000
Fair Value, Inputs, Level 2 \| Revolving credit facility \| Line of Credit
Debt Instrument [Line Items]
Carrying value	210,000,000.0	0
Fair Value	$ 210,000,000.0	$ 0

Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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