Fair Value Measurements (Details) - USD ($) $ in Thousands		Jun. 30, 2020		Dec. 31, 2019		Jun. 30, 2019
1 Percent Convertible Senior Notes [Member]
Fair Value Measurements [Abstract]
Interest rate on convertible senior notes		1.00%		1.00%		1.00%
0.125 Percent Convertible Senior Notes [Member]
Fair Value Measurements [Abstract]
Interest rate on convertible senior notes		0.125%		0.125%
Significant Other Observable Inputs (Level 2) [Member] | 1 Percent Convertible Senior Notes [Member]
Fair Value Measurements [Abstract]
Fair value of convertible notes		$ 340,500
Significant Other Observable Inputs (Level 2) [Member] | 0.125 Percent Convertible Senior Notes [Member]
Fair Value Measurements [Abstract]
Fair value of convertible notes		536,500
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	363,019		$ 418,406
Investment in ProQR Therapeutics N.V.	[2]	2,390		4,506
Total		2,282,177		2,258,985
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		1,150,959	[3]	1,102,568	[4]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		32,400		19,000
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		259,505	[5]	329,404	[6]
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities				800
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		385,651	[7]	363,694	[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		59,700
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		119,744	[8]	40,407	[5]
Recurring Basis [Member] | Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		6,200
Recurring Basis [Member] | Other Municipal Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[5]	909
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		363,019		418,406
Investment in ProQR Therapeutics N.V.		684		0
Total		749,354		782,100
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		385,651		363,694
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
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		0		0
Investment in ProQR Therapeutics N.V.		0		0
Total		1,531,117		1,472,379
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		1,150,959	[3]	1,102,568
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		259,505		329,404
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		119,744		40,407
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other Municipal Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		909
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		0		0
Investment in ProQR Therapeutics N.V.		1,706		4,506
Total		1,706		4,506
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		0		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		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		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		$ 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.

[2] Included in other current assets.

[3] $32.4 million was included in cash and cash equivalents, with the difference included in short-term investments.

[4] $19.0 million was included in cash and cash equivalents, with the difference included in short-term investments.

[5] Included in short-term investments.

[6] $0.8 million was included in cash and cash equivalents, with the difference included in short-term investments.

[7] $59.7 million was included in cash and cash equivalents, with the difference included in short-term investments.

[8] $6.2 million was included in cash and cash equivalents, with the difference included in short-term investments.