Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair value | Level 3
Liabilities:
Kolltan acquisition contingent consideration	$ 15,298	$ 13,779
Fair Value Measurements | Level 2
Assets:
Money market funds and cash equivalents	21,432	15,755
Marketable securities	53,535	69,712
Total financial assets at fair value	74,967	85,467
Fair Value Measurements | Level 3
Liabilities:
Kolltan acquisition contingent consideration	15,298	13,779
Total financial liabilities at fair value	15,298	13,779
Fair Value Measurements | Fair value
Assets:
Money market funds and cash equivalents	21,432	15,755
Marketable securities	53,535	69,712
Total financial assets at fair value	74,967	85,467
Liabilities:
Kolltan acquisition contingent consideration	15,298	13,779
Total financial liabilities at fair value	$ 15,298	$ 13,779