Fair Value Measurements (Assets and Liabilities on a Nonrecurring Basis) (Details) - Level 3 [Member] - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment charge	$ 0
Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Direct investments in privately-held companies, impaired during period	400,000		$ 2,800,000
Estimated fair value of direct investments in privately-held companies	0		1,900,000
Other (Expense) Income, Net [Member] | Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment charge	$ 400,000	$ 1,400,000	$ 900,000