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Organization and Significant Accounting Policies, Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Organization and Significant Accounting Policies [Abstract] |  |  |  |  |
Transfers from Level 1 to Level 2 |  | $ 0 |  | $ 0 |
Transfers from Level 2 to Level 1 |  | 0 |  | 0 |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 1,871,017 |  |  |
Recurring Basis [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Cash equivalents | [1] | 146,281 |  | 86,262 |
Investment in ProQR Therapeutics N.V. | [2] | 1,349 |  |  |
Total |  | 2,017,298 |  | 994,798 |
Recurring Basis [Member] | Corporate Debt Securities [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 1,252,960 | [3] | 647,461 | [4]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 50,200 |  | 11,900 |
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities | [5] | 276,612 |  | 136,325 |
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 260,154 | [6] | 30,818 | [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 |  | 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 |  | 79,942 | [5] | 93,932 | [7]
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 |  |  |  | 3,500 |
Recurring Basis [Member] | Quoted Prices in Active Markets (Level 1) [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Cash equivalents |  | 146,281 |  | 86,262 |
Investment in ProQR Therapeutics N.V. |  | 0 |  |  |
Total |  | 406,435 |  | 117,080 |
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 |  | 260,154 |  | 30,818 |
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] | Significant Other Observable Inputs (Level 2) [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Cash equivalents |  | 0 |  | 0 |
Investment in ProQR Therapeutics N.V. |  | 0 |  |  |
Total |  | 1,609,514 |  | 877,718 |
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 1,252,960 |  | 647,461 |
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 |  | 276,612 |  | 136,325 |
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 |  | 79,942 |  | 93,932 |
Recurring Basis [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Cash equivalents |  | 0 |  |  |
Investment in ProQR Therapeutics N.V. |  | 1,349 |  |  |
Total |  | 1,349 |  | $ 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 |  |  |
[1] | Included in cash and cash equivalents on our consolidated balance sheet.
[2] | Included in other current assets on our consolidated balance sheet.
[3] | $50.2 million included in cash and cash equivalents on our consolidated balance sheet, with the difference included in short-term investments on our consolidated balance sheet.
[4] | $11.9 million included in cash and cash equivalents on our consolidated balance sheet, with the difference included in short-term investments on our consolidated balance sheet.
[5] | Included in short-term investments on our consolidated balance sheet.
[6] | $14.2 million included in cash and cash equivalents on our consolidated balance sheet, with the difference included in short-term investments on our consolidated balance sheet.
[7] | $3.5 million included in cash and cash equivalents on our consolidated balance sheet, with the difference included in short-term investments on our consolidated balance sheet.

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