Investments and Fair Value Measurements - Fair Value Measurements (Details) - Recurring Basis [Member] - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Fair Value Measurements [Abstract]
Money market funds	[1]	$ 74,541	$ 82,343
Total		195,306	249,498
Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		54,404	81,619	[2]
Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities			1,000
Debt Securities Issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[3]	66,361	85,536
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Money market funds	[1]	74,541	82,343
Total		74,541	82,343
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Measurements [Abstract]
Total		120,765	167,155
Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		54,404	81,619	[2]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities Issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[3]	$ 66,361	$ 85,536

[1] Included in cash and cash equivalents on our condensed consolidated balance sheets

[2] At December 31, 2018, $1.0 million was included in cash and cash equivalents on our condensed consolidated balance sheet, with the difference included in short-term investments on our condensed consolidated balance sheet

[3] Included in short-term investments on our condensed consolidated balance sheets