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Fair Value Measurements (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
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1 Percent Convertible Senior Notes [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Interest rate on convertible senior notes |  | 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 |  | $ 314,600 |  |  |
Significant Other Observable Inputs (Level 2) [Member] | 0.125 Percent Convertible Senior Notes [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Fair value of convertible notes |  | 527,300 |  |  |
Recurring Basis [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Cash equivalents | [1] | 372,050 |  | $ 221,125 |
Investment in ProQR Therapeutics N.V. | [2] | 3,198 |  | 2,031 |
Total |  | 1,791,086 |  | 1,745,363 |
Recurring Basis [Member] | Corporate Debt Securities [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities | [3] | 776,776 |  | 846,315 |
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 10,000 |  | 10,000 |
Recurring Basis [Member] | Debt Securities Issued by U.S. Government Agencies [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities | [4] | 177,563 |  | 174,861 |
Recurring Basis [Member] | Debt Securities Issued by the U.S. Treasury [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 293,611 | [4] | 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 | [4] | 156,545 |  | 136,309 |
Recurring Basis [Member] | Other Municipal Debt Securities [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities | [4] | 11,343 |  | 6,225 |
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Cash equivalents |  | 372,050 |  | 221,125 |
Investment in ProQR Therapeutics N.V. |  | 3,198 |  | 2,031 |
Total |  | 668,859 |  | 581,653 |
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 |  | 293,611 |  | 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 |
Investment in ProQR Therapeutics N.V. |  | 0 |  | 0 |
Total |  | 1,122,227 |  | 1,163,710 |
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 776,776 |  | 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 |  | 177,563 |  | 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 |  | 156,545 |  | 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 |  | 11,343 |  | 6,225 |
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 sheet.
[2] | Included in other current assets on our condensed consolidated balance sheet.
[3] | $10.0 million was included in cash and cash equivalents, with the difference included in short-term investments.
[4] | 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.

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