Summary of significant accounting policies - Marketable Securities - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Investments [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 36,641	$ 31,309	$ 7,035	$ 135
Total Money Market Funds And Marketable Securities Fair Value	175,140	222,637
Total Money Market Funds And Marketable Securities Amortized Cost	175,064	222,888
Gross Unrealized Gain	142
Gross Unrealized Loss	66	251
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Due in one year or less	137,777
Restricted Cash and Investments [Abstract]
Restricted cash		300
Marketable Securities.
Schedule of Investments [Line Items]
Marketable debt securities, amortized cost	139,061	193,287
Marketable securities	139,137	193,036
Gross Unrealized Gain	142
Gross Unrealized Loss	66	251
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	139,137	193,036
Money market funds
Schedule of Investments [Line Items]
Cash and Cash Equivalents, at Carrying Value	36,003	29,601
Cash and Cash Equivalents, Fair Value Disclosure	36,003	29,601
U.S. Treasury notes
Schedule of Investments [Line Items]
Marketable debt securities, amortized cost		158,166
Marketable securities		157,981
Gross Unrealized Loss		185
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities		157,981
U.S. Treasury notes | Marketable Securities.
Schedule of Investments [Line Items]
Marketable debt securities, amortized cost	130,237
Marketable securities	130,173
Gross Unrealized Gain	2
Gross Unrealized Loss	66
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	130,173
U. S. Government agency bonds | Marketable Securities.
Schedule of Investments [Line Items]
Marketable debt securities, amortized cost	7,604	35,121
Marketable securities	7,604	35,055
Gross Unrealized Loss		66
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	7,604	$ 35,055
Debt securities | Marketable Securities.
Schedule of Investments [Line Items]
Marketable debt securities, amortized cost	137,841
Marketable securities	137,777
Gross Unrealized Gain	2
Gross Unrealized Loss	66
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	137,777
Equity securities | Marketable Securities.
Schedule of Investments [Line Items]
Marketable debt securities, amortized cost	1,220
Marketable securities	1,360
Gross Unrealized Gain	140
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	$ 1,360