Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair value | Level 3
Liabilities:
Kolltan acquisition contingent consideration	$ 29,800	$ 43,400
Fair Value Measurements | Level 2
Assets:
Money market funds and cash equivalents	27,177	24,061
Marketable securities	83,424	99,139
Total financial assets at fair value	110,601	123,200
Fair Value Measurements | Level 3
Liabilities:
Kolltan acquisition contingent consideration	29,800	43,400
Total financial liabilities at fair value	29,800	43,400
Fair Value Measurements | Fair value
Assets:
Money market funds and cash equivalents	27,177	24,061
Marketable securities	83,424	99,139
Total financial assets at fair value	110,601	123,200
Liabilities:
Kolltan acquisition contingent consideration	29,800	43,400
Total financial liabilities at fair value	$ 29,800	$ 43,400