| **Investments - Schedule of Investments (Details) - USD ($) $ in Thousands** | Jun. 30, 2016 | Dec. 31, 2015 |
|------------------------------------------------------------------------------|---------------|---------------|
| **Schedule of Available-for-sale Securities [Line Items]**                   |          |          |
| Amortized Cost                                                               | $ 223,036     | $ 199,429     |
| Gross Unrealized Gains                                                       | 275           | 0             |
| Gross Unrealized Losses                                                      | 1,388         | 1,117         |
| Fair Value                                                                   | 221,923       | 198,312       |
| U.S. Treasury securities                                                     |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                   |          |          |
| Amortized Cost                                                               | 174,367       | 184,819       |
| Gross Unrealized Gains                                                       | 197           | 0             |
| Gross Unrealized Losses                                                      | 0             | 79            |
| Fair Value                                                                   | 174,564       | 184,740       |
| Corporate debt securities                                                    |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                   |          |          |
| Amortized Cost                                                               | 20,086        | 7,273         |
| Gross Unrealized Gains                                                       | 78            | 0             |
| Gross Unrealized Losses                                                      | 1,152         | 981           |
| Fair Value                                                                   | 19,012        | 6,292         |
| Preferred stock                                                              |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                   |          |          |
| Amortized Cost                                                               | 22,930        | 265           |
| Gross Unrealized Gains                                                       | 0             | 0             |
| Gross Unrealized Losses                                                      | 236           | 57            |
| Fair Value                                                                   | 22,694        | 208           |
| Debt securities                                                              |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                   |          |          |
| Amortized Cost                                                               | 217,383       | 192,357       |
| Gross Unrealized Gains                                                       | 275           | 0             |
| Gross Unrealized Losses                                                      | 1,388         | 1,117         |
| Fair Value                                                                   | 216,270       | 191,240       |
| Money market fund                                                            |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                   |          |          |
| Amortized Cost                                                               | 5,653         |          |
| Gross Unrealized Gains                                                       | 0             |          |
| Gross Unrealized Losses                                                      | 0             |          |
| Fair Value                                                                   | $ 5,653       |          |
| Guaranteed income fund                                                       |          |          |
| **Schedule of Available-for-sale Securities [Line Items]**                   |          |          |
| Amortized Cost                                                               |          | 7,072         |
| Gross Unrealized Gains                                                       |          | 0             |
| Gross Unrealized Losses                                                      |          | 0             |
| Fair Value                                                                   |          | $ 7,072       |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ 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.
+ References
No definition available.
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