| **Investments - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands** | Jun. 30, 2016 | Dec. 31, 2015 |
|------------------------------------------------------------------------------------------|---------------|---------------|
| **Schedule of Available-for-sale Securities [Line Items]**                               |          |          |
| Less Than 12 Months, Fair Value                                                          | $ 40,956      | $ 191,240     |
| Less Than 12 Months, Unrealized Losses                                                   | 1,388         | 1,117         |
| 12 Months or Greater, Fair Value                                                         | 0             | 0             |
| 12 Months or Greater, Unrealized Losses                                                  | 0             | 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                                                          | 18,262        | 6,292         |
| Less Than 12 Months, Unrealized Losses                                                   | 1,152         | 981           |
| 12 Months or Greater, Fair Value                                                         | 0             | 0             |
| 12 Months or Greater, Unrealized Losses                                                  | 0             | 0             |
| Preferred stock                                                                          |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                               |          |          |
| Less Than 12 Months, Fair Value                                                          | 22,694        | 208           |
| Less Than 12 Months, Unrealized Losses                                                   | 236           | 57            |
| 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563
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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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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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