Investments and Fair Value Measurements - Fair Value Measurements (Details) - Recurring Basis [Member] - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Fair Value Measurements [Abstract]
Money market funds	[1]	$ 131,241	$ 82,343
Total		276,615	249,498
Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		84,811	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]	60,563	85,536
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Money market funds	[1]	131,241	82,343
Total		131,241	82,343
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Measurements [Abstract]
Total		145,374	167,155
Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		84,811	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]	$ 60,563	$ 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