Fair Value Measurements (Details) - USD ($) $ in Thousands		Sep. 30, 2019		Dec. 31, 2018		Sep. 30, 2018
Fair Value Measurements [Abstract]
Investment in ProQR Therapeutics N.V.		$ 606		$ 1,349
1 Percent Notes [Member]
Fair Value Measurements [Abstract]
Interest rate on convertible senior notes		1.00%		1.00%		1.00%
Significant Other Observable Inputs (Level 2) [Member] | 1 Percent Notes [Member]
Fair Value Measurements [Abstract]
Fair value of convertible notes		$ 783,700
Recurring Basis [Member]
Fair Value Measurements [Abstract]
Cash equivalents	[1]	202,904		$ 146,281
Investment in ProQR Therapeutics N.V.	[2]	606		1,349
Total		2,195,958		2,017,298
Recurring Basis [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		1,296,598	[3]	1,252,960	[4]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		16,600		50,200
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[5]	283,365		276,612
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		366,272	[6]	260,154	[7]
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		3,000		14,200
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	[5]	43,265		79,942
Recurring Basis [Member] | Other Municipal Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities	[5]	2,948
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		202,904		146,281
Investment in ProQR Therapeutics N.V.		0		0
Total		569,176		406,435
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		366,272		260,154
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		1,626,176		1,609,514
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		1,296,598	[3]	1,252,960
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		283,365		276,612
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		43,265		79,942
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other Municipal Debt Securities [Member]
Fair Value Measurements [Abstract]
Available-for-sale securities		2,948
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements [Abstract]
Cash equivalents		0		0
Investment in ProQR Therapeutics N.V.		606		1,349
Total		606		1,349
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] $16.6 million was included in cash and cash equivalents, with the difference included in short-term investments.

[4] $50.2 million was included in cash and cash equivalents, with the difference included in short-term investments.

[5] Included in short-term investments.

[6] $3.0 million was included in cash and cash equivalents, with the difference included in short-term investments.

[7] $14.2 million was included in cash and cash equivalents, with the difference included in short-term investments.