Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Liabilities:
Kolltan acquisition contingent consideration	$ 12,485	$ 13,779
Fair value | Level 3
Liabilities:
Kolltan acquisition contingent consideration	12,485	13,779
Fair Value Measurements. | Level 2
Assets:
Money market funds and cash equivalents	4,024	15,755
Marketable securities	53,151	69,712
Total financial assets at fair value	57,175	85,467
Fair Value Measurements. | Level 3
Liabilities:
Kolltan acquisition contingent consideration	12,485	13,779
Total financial liabilities at fair value	12,485	13,779
Fair Value Measurements. | Fair value
Assets:
Money market funds and cash equivalents	4,024	15,755
Marketable securities	53,151	69,712
Total financial assets at fair value	57,175	85,467
Liabilities:
Kolltan acquisition contingent consideration	12,485	13,779
Total financial liabilities at fair value	$ 12,485	$ 13,779