Summary of Significant Accounting Policies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)	Jan. 01, 2017 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Fees paid to unaffiliated advisers	$ 46,700	$ 46,700	$ 47,200
Adjustment for adoption of ASU					$ 1,051
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 2016-01
Summary Of Significant Accounting Policies [Line Items]
Adjustment for adoption of ASU				$ 200
Accounting Standards Update 2016-18
Summary Of Significant Accounting Policies [Line Items]
Cash pledged or on deposit of CIP		$ 800	$ 1,000			$ 10,400