Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair value | Level 3
Liabilities:
Kolltan acquisition contingent consideration	$ 15,432	$ 43,400
Fair Value Measurements | Level 2
Assets:
Money market funds and cash equivalents	24,145	24,061
Marketable securities	72,795	99,139
Total financial assets at fair value	96,940	123,200
Fair Value Measurements | Level 3
Liabilities:
Kolltan acquisition contingent consideration	15,432	43,400
Total financial liabilities at fair value	15,432	43,400
Fair Value Measurements | Fair value
Assets:
Money market funds and cash equivalents	24,145	24,061
Marketable securities	72,795	99,139
Total financial assets at fair value	96,940	123,200
Liabilities:
Kolltan acquisition contingent consideration	15,432	43,400
Total financial liabilities at fair value	$ 15,432	$ 43,400