Investments and Fair Value Measurements - Fair Value Measurements (Details) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Fair Value Measurements [Abstract]
Available-for-sale securities		$ 277,982		$ 160,804
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Money market funds	[1]	119,699		285,510
Total		397,681		446,314
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		152,639	[2]	107,735	[3]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		20,000		4,000
Recurring Basis [Member] | Debt Securities Issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[2]	125,343		53,069
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Money market funds	[1]	119,699		285,510
Total		119,699		285,510
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Measurements [Abstract]
Total		277,982		160,804
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		152,639	[2]	107,735	[3]
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	[2]	$ 125,343		$ 53,069

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

[2] Included in short-term investments on our condensed consolidated balance sheet

[3] At March 31, 2020 and December 31, 2019, $20.0 million and $4.0 million, respectively, 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