Summary of significant accounting policies - Marketable Securities - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Investments [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 31,309,000	$ 7,035,000	$ 135,000
Total Money Market Funds And Marketable Securities Fair Value	222,637,000
Marketable securities	193,036,000
Total Money Market Funds And Marketable Securities Amortized Cost	222,888,000
Gross Unrealized Loss	251,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Due in one year or less	163,028,000
Due after one year through two years	30,008,000
Total marketable securities	193,036,000
Marketable Securities [Member]
Schedule of Investments [Line Items]
Marketable Securities, Amortized Cost Basis	193,287,000
Marketable securities	193,036,000
Gross Unrealized Loss	251,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	193,036,000
Money market funds
Schedule of Investments [Line Items]
Cash and Cash Equivalents, at Carrying Value	29,601,000
Cash and Cash Equivalents, Fair Value Disclosure	29,601,000
Deposits and other non-current assets
Restricted Cash and Investments [Abstract]
Restricted cash	314,000	$ 314,000
U.S. Treasury notes | Marketable Securities [Member]
Schedule of Investments [Line Items]
Marketable Securities, Amortized Cost Basis	158,166,000
Marketable securities	157,981,000
Gross Unrealized Loss	185,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	157,981,000
U. S. Government agency bonds | Marketable Securities [Member]
Schedule of Investments [Line Items]
Marketable Securities, Amortized Cost Basis	35,121,000
Marketable securities	35,055,000
Gross Unrealized Loss	66,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	$ 35,055,000