Summary of Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) Segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Fees paid to unaffiliated advisers | $	$ 38.6	$ 40.5	$ 46.7
Minimum
Summary Of Significant Accounting Policies [Line Items]
Exercise period	4 years
Weighted average of useful life	0 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Exercise period	7 years
Weighted average of useful life	5 years
Furniture And Office Equipment | Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	3 years
Furniture And Office Equipment | Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	7 years
Computer equipment and software | Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	3 years
Computer equipment and software | Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	5 years