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
	Mar. 31, 2016 USD ($) item	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Fair value disclosures
Less than 1 year	$ 69,070		$ 66,974
Due in 1 to 2 years	1,251		3,275
Total	70,321		$ 70,249
Impairment losses on its marketable securities	$ 0	$ 0
Investments in marketable securities in unrealized loss position in excess of 12 months | item	0