Fair Value of Financial Instruments Reconciliation of the fair value of recurring Level 3 fair value measurements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Dec. 31, 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		$ 31,416			$ 32,211
Purchases of investments in private companies			(95)	$ 171
Incentive Distribution, Distribution	(70)	$ (86)	(311)	(86)
Loss on change in fair value of limited liability investment			(1,461)	(1,659)
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	$ 10,662	$ 10,662
Transfer from Investments	0	0	0	0
Investments	71,037		71,037			79,839
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	927	2,083	927	$ 1,634	1,397	$ 1,008	$ 939
Real estate investments, at fair value (cost $10,225 and $10,225, respectively)	10,662		10,662			10,662
Investments	12,745		12,745			$ 12,059
Fair Value, Inputs, Level 3 [Member] | Other than Securities Investment [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	12,745	11,589	12,745	11,589
Fair Value, Inputs, Level 3 [Member] | Private Equity Funds, US [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases of investments in private companies	504	35	978	104
Loss on change in fair value of limited liability investment	$ 15	$ (30)	$ 19	$ (30)