Fair Value Measurements (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Restricted short-term investments:
Fair value measurement	$ 43,771	$ 44,489
Money market accounts
Cash equivalents:
Cash equivalents	42,486	42,994
Certificates of deposit
Restricted short-term investments:
Restricted short-term investments	1,285	1,495
Level 1
Restricted short-term investments:
Fair value measurement	43,771	44,489
Level 1 | Money market accounts
Cash equivalents:
Cash equivalents	42,486	42,994
Level 1 | Certificates of deposit
Restricted short-term investments:
Restricted short-term investments	$ 1,285	$ 1,495