Fair Value of Financial Instruments Valuation techniques and significant unobservable inputs utilized in determining fair values (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Limited liability investments, at fair value	$ 31,416		$ 32,211
Investments in private companies, at adjusted cost	4,620		4,870
Real estate investments, at fair value (cost $10,225 and $10,225, respectively)	10,662	$ 10,662	10,662	$ 10,662	$ 10,662	$ 10,662
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Limited liability investments, at fair value	2,083	$ 1,634	1,397	$ 927	$ 1,008	$ 939
Real estate investments, at fair value (cost $10,225 and $10,225, respectively)	$ 10,662		$ 10,662