Investments - Principal Investments in CLOs and Other Investments (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
TCG BDC, Inc.
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	$ 57.6
Corporate Mezzanine Securities Bonds and Warrants
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	$ 600.6	$ 505.2

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Name:	us-gaap_TradingSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant