| **Financial Instruments and Fair Value Measurements (Tables)**                | 3 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 March 31, 2014 were as follows: Level 1 Level 2 Level 3 Total Fair ValueMoney market funds$7,073 $— $— $7,073U.S. Treasury securities209,997 — — 209,997Debt securities:       U.S. Treasury securities224,842 — — 224,842Corporate debt securities— 53,575 — 53,575Preferred stock— 3,373 — 3,373 $441,912 $56,948 $— $498,860The financial instruments measured at fair value on a recurring basis at December 31, 2013 were as follows: Level 1 Level 2 Level 3 Total Fair ValueMoney market funds$1,761 $— $— $1,761Debt securities:       U.S. Treasury securities124,949 — — 124,949Corporate debt securities— 22,023 — 22,023 $126,710 $22,023 $— $148,733 |
| Carrying Amount and Fair Value of Financial Instruments                       | The carrying amount and fair value of the Company’s financial instruments were as follows (the table below excludes financial instruments with carrying values that approximate their fair values): March 31, 2014 December 31, 2013 Carrying value Fair value Level Carrying value Fair value LevelAssets           Pledged treasury securities$26,111 $28,008 1 $26,260 $28,465 1Retained interest investments$9,684 $12,875 3 $9,639 $12,827 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.
+ References
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19190-110258
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