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v3.20.1

Fair Value Measurements (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019 | Mar. 31, 2019
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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 |  | $ 315,300 |  |  |  |
Significant Other Observable Inputs (Level 2) [Member] | 0.125 Percent Convertible Senior Notes [Member] |  |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |  |
Fair value of convertible notes |  | 495,300 |  |  |  |
Recurring Basis [Member] |  |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |  |
Cash equivalents | [1] | 336,371 |  | $ 418,406 |  |
Investment in ProQR Therapeutics N.V. | [2] | 2,089 |  | 4,506 |  |
Total |  | 2,344,798 |  | 2,258,985 |  |
Recurring Basis [Member] | Corporate Debt Securities [Member] |  |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |  |
Available-for-sale securities |  | 1,216,187 | [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 |  | 21,000 |  | 19,000 |  |
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member] |  |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |  |
Available-for-sale securities |  | 334,225 | [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 |  | 6,500 |  |  |  |
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member] |  |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |  |
Available-for-sale securities |  | 420,445 | [7] | 363,694 | [8] |
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 |  | 81,000 |  | 800 |  |
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 | [8] | 34,574 |  | 40,407 |  |
Recurring Basis [Member] | Other Municipal Debt Securities [Member] |  |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |  |
Available-for-sale securities | [8] | 907 |  |  |  |
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] |  |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |  |
Cash equivalents |  | 336,371 |  | 418,406 |  |
Investment in ProQR Therapeutics N.V. |  | 617 |  | 0 |  |
Total |  | 757,433 |  | 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 |  | 420,445 |  | 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 |  | 1,585,893 |  | 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 |  | 1,216,187 | [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 |  | 334,225 |  | 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 |  | 34,574 |  | 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 |  | 907 |  |  |  |
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |  |
Cash equivalents |  | 0 |  | 0 |  |
Investment in ProQR Therapeutics N.V. |  | 1,472 |  | 4,506 |  |
Total |  | 1,472 |  | 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] | $21.0 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] | $6.5 million was included in cash and cash equivalents, with the difference 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] | $81 million was included in cash and cash equivalents, with the difference included in short-term investments.
[8] | Included in short-term investments.

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