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Investments, Summary of Investments (Details) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2021 USD ($) Company | Dec. 31, 2020 USD ($)
Ownership Interests in Private and Public Companies [Abstract] |  |  |
Number of private companies in which there is an equity ownership interest of less than 20% | Company |  | 7 |
Number of public companies in which there is an equity ownership interest of less than 20% | Company |  | 3 |
Summary of Investments [Abstract] |  |  |
Cost |  | $ 1,379,541 | $ 1,533,544
Gross unrealized gains |  | 21,624 | 24,515
Gross unrealized losses |  | (1,528) | (2,821)
Estimated fair value |  | $ 1,399,637 | 1,555,238
Number of publicly traded companies in which there is an investment included in other current assets | Company |  | 2 |
Bicycle [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Gross unrealized gains |  | $ 1,900 |
ProQR [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Gross unrealized losses |  | (700) |
Available-for-sale Securities [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 1,352,029 | 1,513,770
Gross unrealized gains |  | 2,982 | 8,577
Gross unrealized losses |  | (865) | (140)
Estimated fair value |  | 1,354,146 | 1,522,207
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 766,259 | 1,025,454
Gross unrealized gains |  | 1,547 | 2,977
Gross unrealized losses |  | (91) | (71)
Estimated fair value |  | 767,715 | 1,028,360
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 585,770 | 488,316
Gross unrealized gains |  | 1,435 | 5,600
Gross unrealized losses |  | (774) | (69)
Estimated fair value |  | 586,431 | 493,847
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1],[2] | 447,173 | 514,182
Gross unrealized gains |  | 1,338 | 2,194
Gross unrealized losses |  | (50) | (41)
Estimated fair value |  | 448,461 | 516,335
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 366,907 | 325,079
Gross unrealized gains |  | 1,181 | 4,941
Gross unrealized losses |  | (475) | (40)
Estimated fair value |  | 367,613 | 329,980
Debt Securities Issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 83,630 | 94,234
Gross unrealized gains |  | 96 | 354
Gross unrealized losses |  | (7) | (2)
Estimated fair value |  | 83,719 | 94,586
Debt Securities Issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 73,028 | 80,099
Gross unrealized gains |  | 1 | 185
Gross unrealized losses |  | (120) | (9)
Estimated fair value |  | 72,909 | 80,275
Debt Securities Issued by the U.S. Treasury [Member] | Securities with Maturity of One Year or Less [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1],[2] | 89,267 | 307,576
Gross unrealized gains |  | 10 | 233
Gross unrealized losses |  | (2) | (9)
Estimated fair value |  | 89,275 | 307,800
Debt Securities Issued by the U.S. Treasury [Member] | Securities with Maturity of More than One Year [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 98,499 | 50,318
Gross unrealized gains |  | 245 | 383
Gross unrealized losses |  | (88) | (4)
Estimated fair value |  | 98,656 | 50,697
Debt Securities Issued by States of the U.S. and Political Subdivisions of the States [Member] | Securities with Maturity of One Year or Less [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 141,161 | 104,271
Gross unrealized gains |  | 103 | 196
Gross unrealized losses |  | (30) | (12)
Estimated fair value |  | 141,234 | 104,455
Debt Securities Issued by States of the U.S. and Political Subdivisions of the States [Member] | Securities with Maturity of More than One Year [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 41,167 | 31,779
Gross unrealized gains |  | 8 | 91
Gross unrealized losses |  | (65) | (16)
Estimated fair value |  | 41,110 | 31,854
Other Municipal Debt Securities [Member] | Securities with Maturity of One Year or Less [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 5,028 | 5,191
Gross unrealized gains |  | 0 | 0
Gross unrealized losses |  | (2) | (7)
Estimated fair value |  | 5,026 | 5,184
Other Municipal Debt Securities [Member] | Securities with Maturity of More than One Year [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [1] | 6,169 | 1,041
Gross unrealized gains |  | 0 | 0
Gross unrealized losses |  | (26) | 0
Estimated fair value |  | 6,143 | 1,041
Equity Securities [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost |  | 27,512 | 19,774
Gross unrealized gains |  | 18,642 | 15,938
Gross unrealized losses |  | (663) | (2,681)
Estimated fair value |  | 45,491 | 33,031
Equity Securities in Publicly Traded Company [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [3] | 11,897 | 4,712
Gross unrealized gains |  | 1,935 | 0
Gross unrealized losses |  | (663) | (2,681)
Estimated fair value |  | 13,169 | 2,031
Equity Securities in Private Companies [Member] |  |  |
Summary of Investments [Abstract] |  |  |
Cost | [4] | 15,615 | 15,062
Gross unrealized gains |  | 16,707 | 15,938
Gross unrealized losses |  | 0 | 0
Estimated fair value |  | $ 32,322 | $ 31,000
[1] | We hold our available-for-sale securities at amortized cost.
[2] | Includes investments classified as cash equivalents on our condensed consolidated balance sheet.
[3] | Our equity securities included in other current assets consisted of our investments in two publicly traded companies, ProQR and Bicycle, which we classify as Level 1 and Level 3 investments, respectively. We recognize publicly traded equity securities at fair value. In the nine months ended September 30, 2021, we recognized a $1.9 million unrealized gain and a $0.7 million unrealized loss on our condensed consolidated statement of operations related to our investments in Bicycle and ProQR, respectively. In the nine months ended September 30, 2020, our equity securities included in other current assets only consisted of ProQR.
[4] | Our equity securities included in deposits and other assets consisted of our investments in privately held companies. We recognize our private company equity securities at cost minus impairments, plus or minus changes resulting from observable price changes in orderly transactions for the identical or similar investment of the same issuer.

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