| **Financial Instruments and Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($) $ in Thousands** | Dec. 31, 2016 | Dec. 31, 2015 |
|-----------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Restricted investments                                                                                          | $ 5,636       | $ 7,072       |
| Assets, fair value, nonrecurring                                                                                | 0             | 0             |
| Recurring basis                                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 215,907       | 193,206       |
| Debt securities                                                                                                 | 175,725       | 191,240       |
| Restricted investments                                                                                          | 5,636         |          |
| Total                                                                                                           | 397,268       | 391,518       |
| Recurring basis | Level 1                                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 215,907       | 193,206       |
| Debt securities                                                                                                 | 76,965        | 184,740       |
| Restricted investments                                                                                          | 5,636         |          |
| Total                                                                                                           | 298,508       | 377,946       |
| Recurring basis | Level 2                                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 0             | 0             |
| Debt securities                                                                                                 | 98,760        | 6,500         |
| Restricted investments                                                                                          | 0             |          |
| Total                                                                                                           | 98,760        | 13,572        |
| Recurring basis | Level 3                                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 0             | 0             |
| Debt securities                                                                                                 | 0             | 0             |
| Restricted investments                                                                                          | 0             |          |
| Total                                                                                                           | 0             | 0             |
| Recurring basis | Money market fund                                                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 86,236        | 18,233        |
| Restricted investments                                                                                          | 1,410         |          |
| Recurring basis | Money market fund | Level 1                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 86,236        | 18,233        |
| Restricted investments                                                                                          | 1,410         |          |
| Recurring basis | Money market fund | Level 2                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 0             | 0             |
| Restricted investments                                                                                          | 0             |          |
| Recurring basis | Money market fund | Level 3                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 0             | 0             |
| Restricted investments                                                                                          | 0             |          |
| Recurring basis | Commercial paper                                                                              |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 129,671       | 174,973       |
| Recurring basis | Commercial paper | Level 1                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 129,671       | 174,973       |
| Recurring basis | Commercial paper | Level 2                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 0             | 0             |
| Recurring basis | Commercial paper | Level 3                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Cash equivalents                                                                                                | 0             | 0             |
| Recurring basis | U.S. Treasury securities                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 |          | 184,740       |
| Recurring basis | U.S. Treasury securities | Level 1                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 |          | 184,740       |
| Recurring basis | U.S. Treasury securities | Level 2                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 |          | 0             |
| Recurring basis | U.S. Treasury securities | Level 3                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 |          | 0             |
| Recurring basis | Corporate debt securities                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 | 139,132       | 6,292         |
| Recurring basis | Corporate debt securities | Level 1                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 | 57,788        | 0             |
| Recurring basis | Corporate debt securities | Level 2                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 | 81,344        | 6,292         |
| Recurring basis | Corporate debt securities | Level 3                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 | 0             | 0             |
| Recurring basis | Preferred stock                                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 | 36,593        | 208           |
| Recurring basis | Preferred stock | Level 1                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 | 19,177        | 0             |
| Recurring basis | Preferred stock | Level 2                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 | 17,416        | 208           |
| Recurring basis | Preferred stock | Level 3                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Debt securities                                                                                                 | 0             | 0             |
| Recurring basis | Short-term bond                                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Restricted investments                                                                                          | 4,226         |          |
| Recurring basis | Short-term bond | Level 1                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Restricted investments                                                                                          | 4,226         |          |
| Recurring basis | Short-term bond | Level 2                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Restricted investments                                                                                          | 0             |          |
| Recurring basis | Short-term bond | Level 3                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Restricted investments                                                                                          | $ 0           |          |
| Recurring basis | Investments                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Restricted investments                                                                                          |          | 7,072         |
| Recurring basis | Investments | Level 1                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Restricted investments                                                                                          |          | 0             |
| Recurring basis | Investments | Level 2                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Restricted investments                                                                                          |          | 7,072         |
| Recurring basis | Investments | Level 3                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                |          |          |
| Restricted investments                                                                                          |          | $ 0           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ References
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| - Definition
This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a nonrecurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.
+ References
No definition available.
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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
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
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No definition available.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.
+ References
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| - Definition
Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.
+ References
No definition available.
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