Summary of Significant Accounting Policies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments (in segments) | Segment	1
Fees paid to unaffiliated advisers	$ 40,500	$ 46,700	$ 46,700
Operating lease right of use assets	21,000
Operating lease liabilities	$ 29,402
Minimum
Summary Of Significant Accounting Policies [Line Items]
Exercise period	4 years
Weighted average of useful life	5 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Exercise period	7 years
Weighted average of useful life	16 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
Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease right of use assets				$ 20,500
Operating lease liabilities				$ 28,600