Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair value | Level 3
Liabilities:
Kolltan acquisition contingent consideration	$ 13,779	$ 43,400
Fair Value Measurements | Level 2
Assets:
Money market funds and cash equivalents	15,755	24,061
Marketable securities	69,712	99,139
Total financial assets at fair value	85,467	123,200
Fair Value Measurements | Level 3
Liabilities:
Kolltan acquisition contingent consideration	13,779	43,400
Total financial liabilities at fair value	13,779	43,400
Fair Value Measurements | Fair value
Assets:
Money market funds and cash equivalents	15,755	24,061
Marketable securities	69,712	99,139
Total financial assets at fair value	85,467	123,200
Liabilities:
Kolltan acquisition contingent consideration	13,779	43,400
Total financial liabilities at fair value	$ 13,779	$ 43,400