Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Private equity investments, included in non-marketable securities	$ 85,659	$ 55,752
Fair Value Hierarchy, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Auction rate securities, available for sale	14,200
Private equity investments, included in non-marketable securities	$ 85,659	$ 55,752