Fair Value Measurements - Financial Assets and Liabilities Carried at Fair Value on Recurring Basis (Details) - Recurring [Member] - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Assets:
Long-term equity securities		$ 392
Money Market Funds [Member]
Assets:
Money market funds	[1]		$ 34,907
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Money Market Funds [Member]
Assets:
Money market funds	[1]		$ 34,907
Significant Unobservable Inputs (Level 3) [Member]
Assets:
Long-term equity securities		$ 392

[1] Included in cash and cash equivalents