Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair value | Level 3
Liabilities:
Kolltan acquisition contingent consideration	$ 43,400	$ 44,200
Fair Value Measurements | Level 2
Assets:
Money market funds and cash equivalents	24,061	20,445
Marketable securities	99,139	147,315
Total financial assets at fair value	123,200	167,760
Fair Value Measurements | Level 3
Liabilities:
Kolltan acquisition contingent consideration	43,400	44,200
Total financial liabilities at fair value	43,400	44,200
Fair Value Measurements | Fair value
Assets:
Money market funds and cash equivalents	24,061	20,445
Marketable securities	99,139	147,315
Total financial assets at fair value	123,200	167,760
Liabilities:
Kolltan acquisition contingent consideration	43,400	44,200
Total financial liabilities at fair value	$ 43,400	$ 44,200