Investments and Fair Value Measurements, Fair Value Measurements (Details) - USD ($)		Jun. 30, 2018		Dec. 31, 2017
Fair Value Measurements [Abstract]
Transfers from Level 1 to Level 2		$ 0		$ 0
Transfers from Level 2 to Level 1		0		0
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Money market funds	[1]	193,752,000		48,430,000
Total		364,761,000		250,193,000
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		89,211,000	[2]	140,460,000	[3]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		6,000,000
Recurring Basis [Member] | Debt Securities Issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[3]	81,798,000		61,303,000
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Money market funds		193,752,000		48,430,000
Total		193,752,000		48,430,000
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0		0
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt Securities Issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0		0
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Measurements [Abstract]
Money market funds		0		0
Total		171,009,000		201,763,000
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		89,211,000		140,460,000
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		81,798,000		61,303,000
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements [Abstract]
Total		$ 0		$ 0

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

[2] At June 30, 2018, $6.0 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.