Cash Equivalents and Equity Securities - Components of Cash Equivalents and Equity Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 53,039	$ 31,219	$ 19,240
Equity securities, adjusted cost basis	563
Equity securities, gross unrealized gains	25
Equity securities, gross unrealized losses	0
Equity securities, estimated fair value	588
Equity securities, adjusted cost	31,788
Equity securities, adjusted cost		563
Equity securities, gross unrealized gains		108
Equity securities, gross unrealized losses		0
Equity securities, estimated fair value		671
Adjusted Cost		7,341
Gross unrealized gains		108
Gross unrealized losses		0
Estimated Fair Value	31,813	7,449
Money market funds
Equity Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 31,225	$ 6,778