| **Financial Instruments and Fair Value Measurements (Tables)**                | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
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| **Fair Value Disclosures [Abstract]**                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
| Schedule of Financial Instruments Measured at Fair Value on a Recurring Basis | The financial instruments measured at fair value on a recurring basis at December 31, 2016 were as follows: Level 1 Level 2 Level 3 Total Fair ValueCash equivalents:       Money market funds$86,236 $— $— $86,236Commercial paper129,671 — — 129,671 215,907 — — 215,907Investments:       Corporate debt securities57,788 81,344 — 139,132Preferred stock19,177 17,416 — 36,593 76,965 98,760 — 175,725Restricted investments:       Short-term bond4,226 — — 4,226Money market fund1,410 — — 1,410 5,636 — — 5,636 $298,508$98,760$—$397,268The financial instruments measured at fair value on a recurring basis at December 31, 2015 were as follows: Level 1 Level 2 Level 3 Total Fair ValueCash equivalents:       Money market funds$18,233 $— $— $18,233Commercial paper174,973 — — 174,973 193,206——193,206Investments:       U.S. treasury securities184,740 — — 184,740Corporate debt securities— 6,292 — 6,292Preferred stock— 208 — 208 184,740 6,500 — 191,240Restricted investments:       Guaranteed income fund— 7,072 — 7,072 $377,946 $13,572 $— $391,518 |
| Carrying Amount and Fair Value of Financial Instruments                       | The carrying amount and fair value of the Company’s financial instruments were as follows: December 31, 2016 December 31, 2015 Carrying value Fair value Level Carrying value Fair value LevelAssets           Retained interest investments$10,635 $13,669 3 $10,246 $13,333 3Investments held by SPEs:           Time deposit$200,000 $200,000 3 $200,000 $200,000 3U.S. Treasury securities and cash equivalents$8,590 $8,398 1 $8,785 $9,033 1Liabilities           Senior Notes held by SPE$176,310 $199,691 3 $176,094 $178,035 3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |

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| - Definition
Tabular disclosure of the fair value of financial instruments, including financial assets and financial liabilities, and the measurements of those instruments, assets, and liabilities.
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No definition available.
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| - Definition
Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3).
+ References
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