Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Roll-forward of acquisition-related contingent consideration
Changes in fair value	$ 3,400
Assets transferred between the fair value measurement classifications		$ 0
Liabilities transferred between the fair value measurement classifications		0
Fair Value Measurements | Level 2
Assets:
Money market funds and cash equivalents		28,572	$ 20,445
Marketable securities		124,526	147,315
Total financial assets at fair value		153,098	167,760
Fair Value Measurements | Level 3
Liabilities:
Kolltan acquisition contingent consideration	44,200	47,600	44,200
Total financial liabilities at fair value		47,600	44,200
Roll-forward of acquisition-related contingent consideration
Balance at beginning of period	44,200
Balance at end of period	47,600
Fair Value Measurements | Fair value
Assets:
Money market funds and cash equivalents		28,572	20,445
Marketable securities		124,526	147,315
Total financial assets at fair value		153,098	167,760
Liabilities:
Kolltan acquisition contingent consideration	44,200	47,600	44,200
Total financial liabilities at fair value		47,600	44,200
Roll-forward of acquisition-related contingent consideration
Balance at beginning of period	44,200
Balance at end of period	47,600
Fair Value Measurements | Fair value | Level 3
Liabilities:
Kolltan acquisition contingent consideration	44,200	$ 47,600	$ 44,200
Roll-forward of acquisition-related contingent consideration
Balance at beginning of period	44,200
Changes in fair value	3,400
Balance at end of period	$ 47,600