Fair Value of Financial Instruments Valuation techniques and significant unobservable inputs utilized in determining fair values for the Company's investments that are categorized as Level 3 (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Alternative Investment	$ 31,564		$ 32,211
Investments in private companies, at adjusted cost	4,615		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]
Alternative Investment	2,330	$ 2,083	1,397	$ 1,503	$ 927	$ 939
Real estate investments, at fair value (cost $10,225 and $10,225, respectively)	$ 10,662		$ 10,662