Fair Value Measurements (Details) - Money Market Funds [Member] - USD ($) $ in Thousands |
Jun. 30, 2015 |
Dec. 31, 2014 |
|---|---|---|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Investments fair value | $ 698,608 | $ 667,210 |
| Level 1 [Member] | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Investments fair value | 698,608 | 667,210 |
| Level 2 [Member] | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Investments fair value | 0 | 0 |
| Level 3 [Member] | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Investments fair value | $ 0 | $ 0 |