Fair Value Measurements (Details) - USD ($) $ in Thousands		Sep. 30, 2021		Dec. 31, 2020		Sep. 30, 2020
1% Notes [Member]
Fair Value Measurements [Abstract]
Interest rate on convertible senior notes		1.00%		1.00%		1.00%
0.125% Notes [Member]
Fair Value Measurements [Abstract]
Interest rate on convertible senior notes		0.125%		0.125%		0.125%
0% Notes [Member]
Fair Value Measurements [Abstract]
Interest rate on convertible senior notes		0.00%
Significant Other Observable Inputs (Level 2) [Member] | 1% Notes [Member]
Fair Value Measurements [Abstract]
Fair value of convertible notes		$ 61,900
Significant Other Observable Inputs (Level 2) [Member] | 0.125% Notes [Member]
Fair Value Measurements [Abstract]
Fair value of convertible notes		504,300
Significant Other Observable Inputs (Level 2) [Member] | 0% Notes [Member]
Fair Value Measurements [Abstract]
Fair value of convertible notes		581,400
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	545,339		$ 221,125
Total		1,912,654		1,745,363
Recurring Basis [Member] | Bicycle Therapeutics plc [Member]
Fair Value Measurements [Abstract]
Investment	[2]	9,120
Recurring Basis [Member] | ProQR Therapeutics N.V. [Member]
Fair Value Measurements [Abstract]
Investment	[2]	4,049		2,031
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		816,074	[3]	846,315	[4]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities				10,000
Recurring Basis [Member] | Debt Securities Issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[3]	156,628		174,861
Recurring Basis [Member] | Debt Securities Issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		187,931	[3]	358,497	[5]
Recurring Basis [Member] | Debt Securities Issued by the U.S. Treasury [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities				17,500
Recurring Basis [Member] | Debt Securities Issued by States of the U.S. and Political Subdivisions of the States [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		182,344		136,309	[3]
Recurring Basis [Member] | Other Municipal Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[3]	11,169		6,225
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		545,339		221,125
Total		737,319		581,653
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Bicycle Therapeutics plc [Member]
Fair Value Measurements [Abstract]
Investment		0
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | ProQR Therapeutics N.V. [Member]
Fair Value Measurements [Abstract]
Investment		4,049		2,031
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] | Quoted Prices in Active Markets (Level 1) [Member] | Debt Securities Issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		187,931		358,497
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Debt Securities Issued by States of the U.S. and Political Subdivisions of the States [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0		0
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] | Other Municipal Debt Securities [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]
Cash equivalents		0		0
Total		1,166,215		1,163,710
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Bicycle Therapeutics plc [Member]
Fair Value Measurements [Abstract]
Investment		0
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | ProQR Therapeutics N.V. [Member]
Fair Value Measurements [Abstract]
Investment		0		0
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		816,074		846,315
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		156,628		174,861
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities Issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0		0
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities Issued by States of the U.S. and Political Subdivisions of the States [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		182,344		136,309
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other Municipal Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		11,169		6,225
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		0
Total		9,120		$ 0
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Bicycle Therapeutics plc [Member]
Fair Value Measurements [Abstract]
Investment		9,120
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | ProQR Therapeutics N.V. [Member]
Fair Value Measurements [Abstract]
Investment		0
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities Issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities Issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities Issued by States of the U.S. and Political Subdivisions of the States [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other Municipal Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		$ 0

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

[2] Included in other current assets on our condensed consolidated balance sheet.

[3] Included in short-term investments.

[4] $10.0 million was included in cash and cash equivalents, with the difference included in short-term investments.

[5] $17.5 million 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.