| 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 September 30, 2013 and December 31, 2012 (in thousands): | | | | | | | | | | | | | | | | | | Fair Value as of September 30, 2013 | | Level 1 | | Level 2 | | Level 3 | | Total | Assets: | | | | | | | | Money market and demand accounts (a) | $ | 518,352 |
| | $ | — |
| | $ | — |
| | $ | 518,352 |
| Commercial paper (b) | — |
| | 188,060 |
| | — |
| | 188,060 |
| U.S. government securities | — |
| | 37,463 |
| | — |
| | 37,463 |
| Corporate bonds, asset backed and other securities | 1,766 |
| | 11,201 |
| | — |
| | 12,967 |
| | $ | 520,118 |
| | $ | 236,724 |
| | $ | — |
| | $ | 756,842 |
|
______________________________ | | (a) | Included within cash and cash equivalents. |
| | (b) | Includes $37.2 million of commercial paper that is included within cash and cash equivalents. |
| | | | | | | | | | | | | | | | | | Fair Value as of December 31, 2012 | | Level 1 | | Level 2 | | Level 3 | | Total | Assets: | | | | | | | | Money market and demand accounts (a) | $ | 261,899 |
| | $ | — |
| | $ | — |
| | $ | 261,899 |
| Mutual and exchange traded funds | 100,682 |
| | — |
| | — |
| | 100,682 |
| Commercial paper (b) | — |
| | 150,868 |
| | — |
| | 150,868 |
| U.S. government securities | — |
| | 50,560 |
| | — |
| | 50,560 |
| Corporate bonds, asset backed and other securities | — |
| | 13,270 |
| | — |
| | 13,270 |
| | $ | 362,581 |
| | $ | 214,698 |
| | $ | — |
| | $ | 577,279 |
|
______________________________ | | (a) | Included within cash and cash equivalents. |
| | (b) | Includes $87.9 million of commercial paper that is included within cash and cash equivalents. |
|