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Fair Value Measurements, Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Fair Value Measurements [Abstract] |  |  |  |  |
Transfers from Level 1 to Level 2 |  | $ 0 |  |  |
Transfers from Level 2 to Level 1 |  | 0 |  |  |
Recurring Basis [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Cash equivalents | [1] | 395,649 |  | $ 86,262 |
Total |  | 1,894,487 |  | 994,798 |
Recurring Basis [Member] | Corporate Debt Securities [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 1,078,960 | [2] | 647,461 | [3]
Recurring Basis [Member] | Corporate Debt Securities [Member] | Cash and Cash Equivalents [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 19,700 |  |  |
Recurring Basis [Member] | Debt Securities issued by U.S. Government Agencies [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities | [3] | 175,167 |  | 136,325 |
Recurring Basis [Member] | Debt Securities issued by the U.S. Treasury [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities | [3] | 164,053 |  | 30,818 |
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 |  | 80,658 | [3] | 93,932 | [4]
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 |  | 395,649 |  | 86,262 |
Total |  | 559,702 |  | 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 |  | 164,053 |  | 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 |
Total |  | 1,334,785 |  | 877,718 |
Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member] |  |  |  |  |
Fair Value Measurements [Abstract] |  |  |  |  |
Available-for-sale securities |  | 1,078,960 | [2] | 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 |  | 175,167 |  | 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 |  | $ 80,658 |  | $ 93,932 |
[1] | Included in cash and cash equivalents on our condensed consolidated balance sheet.
[2] | At September 30, 2018, $19.7 million was 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.
[3] | Included in short-term investments on our condensed consolidated balance sheet.
[4] | At December 31, 2017, $3.5 million was 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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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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