Fair Value Measurements (Details) - Money market funds - USD ($) $ in Thousands |
Jun. 30, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Investments Fair Value | $ 952,661 | $ 872,161 |
| Level 1 | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Investments Fair Value | 952,661 | 872,161 |
| Level 2 | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Investments Fair Value | 0 | 0 |
| Level 3 | ||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Investments Fair Value | $ 0 | $ 0 |