| Fair Value, Measurement Inputs, Disclosure [Table Text Block] |
Our financial assets that are accounted for at fair value on a recurring basis are presented in the tables below as of June 30, 2015 and December 31, 2014 (in thousands): | | | | | | | | | | | | | | | | | | Fair Value as of June 30, 2015 | | Level 1 | | Level 2 | | Level 3 | | Total | Assets: | | | | | | | | Money market and demand accounts (a) | $ | 357,090 |
| | $ | — |
| | $ | — |
| | $ | 357,090 |
| Commercial paper (b) | — |
| | 300,004 |
| | — |
| | 300,004 |
| U.S. government securities | — |
| | 209,071 |
| | — |
| | 209,071 |
| Corporate bonds, asset backed and other securities | 593 |
| | 40,837 |
| | — |
| | 41,430 |
| | $ | 357,683 |
| | $ | 549,912 |
| | $ | — |
| | $ | 907,595 |
|
______________________________ | | (a) | Included within cash and cash equivalents. |
| | (b) | Includes $149.9 million of commercial paper that is included within cash and cash equivalents. |
| | | | | | | | | | | | | | | | | | Fair Value as of December 31, 2014 | | Level 1 | | Level 2 | | Level 3 | | Total | Assets: | | | | | | | | Money market and demand accounts (a) | $ | 307,995 |
| | $ | — |
| | $ | — |
| | $ | 307,995 |
| Commercial paper (b) | — |
| | 207,449 |
| | — |
| | 207,449 |
| U.S. government securities | — |
| | 151,618 |
| | — |
| | 151,618 |
| Corporate bonds, asset backed and other securities | 671 |
| | 36,195 |
| | — |
| | 36,866 |
| | $ | 308,666 |
| | $ | 395,262 |
| | $ | — |
| | $ | 703,928 |
|
______________________________ | | (a) | Included within cash and cash equivalents. |
| | (b) | Includes $120.6 million of commercial paper that is included within cash and cash equivalents. |
|