| **Investments - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands** | Dec. 31, 2016 | Dec. 31, 2015 |
|------------------------------------------------------------------------------------------|---------------|---------------|
| **Schedule of Available-for-sale Securities [Line Items]**                               |          |          |
| Less Than 12 Months, Fair Value                                                          | $ 68,742      | $ 191,240     |
| Less Than 12 Months, Unrealized Losses                                                   | 1,416         | 1,117         |
| 12 Months or Greater, Fair Value                                                         | 7,124         | 0             |
| 12 Months or Greater, Unrealized Losses                                                  | 470           | 0             |
| U.S. Treasury securities                                                                 |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                               |          |          |
| Less Than 12 Months, Fair Value                                                          | 0             | 184,740       |
| Less Than 12 Months, Unrealized Losses                                                   | 0             | 79            |
| 12 Months or Greater, Fair Value                                                         | 0             | 0             |
| 12 Months or Greater, Unrealized Losses                                                  | 0             | 0             |
| Corporate debt securities                                                                |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                               |          |          |
| Less Than 12 Months, Fair Value                                                          | 64,516        | 6,292         |
| Less Than 12 Months, Unrealized Losses                                                   | 1,410         | 981           |
| 12 Months or Greater, Fair Value                                                         | 6,971         | 0             |
| 12 Months or Greater, Unrealized Losses                                                  | 359           | 0             |
| Preferred stock                                                                          |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                               |          |          |
| Less Than 12 Months, Fair Value                                                          | 0             | 208           |
| Less Than 12 Months, Unrealized Losses                                                   | 0             | 57            |
| 12 Months or Greater, Fair Value                                                         | 153           | 0             |
| 12 Months or Greater, Unrealized Losses                                                  | 111           | 0             |
| Short-term bond                                                                          |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                               |          |          |
| Less Than 12 Months, Fair Value                                                          | 4,226         | 0             |
| Less Than 12 Months, Unrealized Losses                                                   | 6             | 0             |
| 12 Months or Greater, Fair Value                                                         | 0             | 0             |
| 12 Months or Greater, Unrealized Losses                                                  | $ 0           | $ 0           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
No definition available.
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