Fair Value Measurements (Details) - Fair Value Measurements - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Level 2
Assets:
Money market funds and cash equivalents	$ 42,032	$ 20,445
Marketable securities	85,795	147,315
Total financial assets at fair value	127,827	167,760
Level 3
Liabilities:
Kolltan acquisition contingent consideration	44,000	44,200
Total financial liabilities at fair value	44,000	44,200
Fair value
Assets:
Money market funds and cash equivalents	42,032	20,445
Marketable securities	85,795	147,315
Total financial assets at fair value	127,827	167,760
Liabilities:
Kolltan acquisition contingent consideration	44,000	44,200
Total financial liabilities at fair value	$ 44,000	$ 44,200