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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
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.