Summary of Cash, Cash Equivalents, Short-Term investments, and Restricted Cash and Investments - Maturities of marketable securities and additional information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) item
Fair value disclosures
Less than 1 year	$ 46,645		$ 46,645		$ 66,974
Due in 1 to 2 years	5,784		5,784		3,275
Total	52,429		52,429		$ 70,249
Impairment losses on its marketable securities	$ 0	$ 0	$ 0	$ 0
Investments in marketable securities in unrealized loss position in excess of 12 months | item	0		0		0