Investments and Fair Value Measurements - Fair Value Measurements (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Fair Value Measurements [Abstract]
Available-for-sale securities		$ 123,362
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Money market funds	[1]	124,972	$ 82,343
Total		248,334	249,498
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		73,843	81,619	[2]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities			1,000
Recurring Basis [Member] | Debt Securities Issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[3]	49,519	85,536
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Money market funds	[1]	124,972	82,343
Total		124,972	82,343
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Measurements [Abstract]
Total		123,362	167,155
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		73,843	81,619	[2]
Recurring Basis [Member] | 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]	$ 49,519	$ 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