Fair Value of Financial Instruments Investments Measured Using the Net Asset Value per Share Practical Expedient (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Gain (Loss) on Marketable Securities, Cost Method Investments, and Other Investments	$ (155,000)	$ (1,512,000)	$ (155,000)	$ (1,265,000)
Limited liability investments, at fair value	31,416,000		31,416,000		$ 32,211,000
Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Limited liability investments, at fair value	20,835,000		20,835,000		$ 20,500,000
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Gain (Loss) on Marketable Securities, Cost Method Investments, and Other Investments	154,000
Equity Method Investment, Other than Temporary Impairment	$ 0	$ 1,500,000	$ 0	$ 1,500,000