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Fair Value Measurement (Tables) | 12 Months Ended
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Dec. 31, 2017
Fair Value Disclosures [Abstract] |
Summary of Investments Measured at Fair Value on Recurring Basis | The following table summarizes the Company’s investments at December 31, 2017 and 2016 measured at fair value on a recurring basis (in thousands):
|  | December 31, 2017 |  |  | December 31, 2016 |
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Gross |  |  |  |  |
|  |  |  |  |  | Gross |  |  |  |  |  |  |  |  |  |  | Unrealized |  |  |  |  |
|  |  |  |  |  | Unrealized |  |  | Recorded |  |  |  |  |  |  | Gains |  |  | Recorded |
|  | Cost |  |  | (Losses) |  |  | Basis |  |  | Cost |  |  | (Losses) |  |  | Basis |
Current: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash equivalents: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Money market funds |  | $ | 121 |  |  | $ | — |  |  | $ | 121 |  |  | $ | 1,943 |  |  | $ | — |  |  | $ | 1,943 |
U.S. treasuries |  |  | 100 |  |  |  | — |  |  |  | 100 |  |  |  | — |  |  |  | — |  |  |  | — |
Total cash equivalents |  |  | 221 |  |  |  | — |  |  |  | 221 |  |  |  | 1,943 |  |  |  | — |  |  |  | 1,943 |
Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 400 |  |  |  | — |  |  |  | 400 |  |  |  | 225 |  |  |  | (1 | ) |  |  | 224 |
U.S. treasuries |  |  | 199 |  |  |  | — |  |  |  | 199 |  |  |  | — |  |  |  | — |  |  |  | — |
U.S. government agency securities |  |  | 65 |  |  |  | — |  |  |  | 65 |  |  |  | — |  |  |  | — |  |  |  | — |
Municipal bonds |  |  | 10 |  |  |  | — |  |  |  | 10 |  |  |  | 713 |  |  |  | 1 |  |  |  | 714 |
Certificates of deposit |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 4,737 |  |  |  | — |  |  |  | 4,737 |
Total current investments |  |  | 674 |  |  |  | — |  |  |  | 674 |  |  |  | 5,675 |  |  |  | — |  |  |  | 5,675 |
Long term: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage backed securities |  |  | 577 |  |  |  | (5 | ) |  |  | 572 |  |  |  | — |  |  |  | — |  |  |  | — |
Corporate bonds |  |  | 419 |  |  |  | (2 | ) |  |  | 417 |  |  |  | 567 |  |  |  | (1 | ) |  |  | 566 |
U.S. treasuries |  |  | 202 |  |  |  | (2 | ) |  |  | 200 |  |  |  | — |  |  |  | — |  |  |  | — |
Asset backed securities |  |  | 10 |  |  |  | — |  |  |  | 10 |  |  |  | — |  |  |  | — |  |  |  | — |
Municipal bonds |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 76 |  |  |  | — |  |  |  | 76 |
Total long term investments |  |  | 1,208 |  |  |  | (9 | ) |  |  | 1,199 |  |  |  | 643 |  |  |  | (1 | ) |  |  | 642 |
Restricted cash and investments (1): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 184,808 |  |  |  | (953 | ) |  |  | 183,855 |  |  |  | 2,886 |  |  |  | 2 |  |  |  | 2,888 |
Mortgage backed securities |  |  | 86,240 |  |  |  | (595 | ) |  |  | 85,645 |  |  |  | — |  |  |  | — |  |  |  | — |
U.S. treasuries |  |  | 45,833 |  |  |  | (143 | ) |  |  | 45,690 |  |  |  | 834 |  |  |  | — |  |  |  | 834 |
U.S. government agency securities |  |  | 38,168 |  |  |  | (222 | ) |  |  | 37,946 |  |  |  | — |  |  |  | — |  |  |  | — |
Commercial paper |  |  | 18,973 |  |  |  | — |  |  |  | 18,973 |  |  |  | — |  |  |  | — |  |  |  | — |
Money market funds |  |  | 16,018 |  |  |  | — |  |  |  | 16,018 |  |  |  | 284,593 |  |  |  | — |  |  |  | 284,593 |
Municipal bonds |  |  | 472 |  |  |  | (14 | ) |  |  | 458 |  |  |  | 2,069 |  |  |  | (6 | ) |  |  | 2,063 |
Certificates of deposit |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 6,047 |  |  |  | — |  |  |  | 6,047 |
Total restricted cash and investments |  |  | 390,512 |  |  |  | (1,927 | ) |  |  | 388,585 |  |  |  | 296,429 |  |  |  | (4 | ) |  |  | 296,425 |
Total investments |  | $ | 392,615 |  |  | $ | (1,936 | ) |  | $ | 390,679 |  |  | $ | 304,690 |  |  | $ | (5 | ) |  | $ | 304,685 |
(1) Included in restricted cash and investments within the consolidated balance sheet as of December 31, 2017 is restricted cash and long term workers' compensation deposits of $6.3 million, which is excluded from the table above. Restricted cash and investments are classified as current and noncurrent on the balance sheet based on the nature of the restriction.
Summary of Assets Measured at Fair Value on Recurring Basis | The following table summarizes the Company's financial assets measured at fair value on a recurring basis by fair value hierarchy level (in thousands):
|  | December 31, 2017 |  |  | December 31, 2016 |
|  | Total |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Total |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
|  | Recorded |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Recorded |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
|  | Basis |  |  | Level 1 |  |  | Level 2 |  |  | Level 3 |  |  | Other (1) |  |  | Basis |  |  | Level 1 |  |  | Level 2 |  |  | Level 3 |  |  | Other (1) |
Cash equivalents: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Money market funds |  | $ | 121 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 121 |  |  | $ | 1,943 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 1,943 |
U.S. treasuries |  |  | 100 |  |  |  | — |  |  |  | 100 |  |  |  | — |  |  |  |  |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
Investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 817 |  |  |  | — |  |  |  | 817 |  |  |  | — |  |  |  | — |  |  |  | 790 |  |  |  | — |  |  |  | 790 |  |  |  | — |  |  |  | — |
Mortgage backed securities |  |  | 572 |  |  |  | — |  |  |  | 572 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
U.S. treasuries |  |  | 399 |  |  |  | — |  |  |  | 399 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
U.S. government agency securities |  |  | 65 |  |  |  | — |  |  |  | 65 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
Municipal bonds |  |  | 10 |  |  |  | — |  |  |  | 10 |  |  |  | — |  |  |  | — |  |  |  | 790 |  |  |  | — |  |  |  | 790 |  |  |  | — |  |  |  | — |
Asset backed securities |  |  | 10 |  |  |  | — |  |  |  | 10 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  |  |  |  |  | — |  |  |  | — |
Certificates of deposit |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 4,737 |  |  |  | — |  |  |  | 4,737 |  |  |  | — |  |  |  | — |
Restricted cash and investments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  |  | 183,855 |  |  |  | — |  |  |  | 183,855 |  |  |  | — |  |  |  | — |  |  |  | 2,888 |  |  |  | — |  |  |  | 2,888 |  |  |  | — |  |  |  | — |
Mortgage backed securities |  |  | 85,645 |  |  |  | — |  |  |  | 85,645 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
U.S. treasuries |  |  | 45,690 |  |  |  | — |  |  |  | 45,690 |  |  |  | — |  |  |  | — |  |  |  | 834 |  |  |  | — |  |  |  | 834 |  |  |  | — |  |  |  | — |
U.S. government agency securities |  |  | 37,946 |  |  |  | — |  |  |  | 37,946 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
Commercial paper |  |  | 18,973 |  |  |  | — |  |  |  | 18,973 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
Money market funds |  |  | 16,018 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 16,018 |  |  |  | 284,593 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 284,593 |
Municipal bonds |  |  | 458 |  |  |  | — |  |  |  | 458 |  |  |  | — |  |  |  | — |  |  |  | 2,063 |  |  |  | — |  |  |  | 2,063 |  |  |  | — |  |  |  | — |
Certificates of deposit |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 6,047 |  |  |  | — |  |  |  | 6,047 |  |  |  | — |  |  |  | — |
Total investments |  | $ | 390,679 |  |  | $ | — |  |  | $ | 374,540 |  |  | $ | — |  |  | $ | 16,139 |  |  | $ | 304,685 |  |  | $ | — |  |  | $ | 18,149 |  |  | $ | — |  |  | $ | 286,536 |
(1) Investments in money market funds measured at fair value using the net asset value per share practical expedient are not subject to hierarchy level classification disclosure. The Company invests in money market funds that seek to maintain a stable net asset value. These investments include commingled funds that comprise high-quality short-term securities representing liquid debt and monetary instruments where the redemption value is likely to be the fair value. Redemption is permitted daily without written notice.

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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=66048111&loc=d3e19190-110258 + Details
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