NATURE OF BUSINESS AND OVERVIEW - (Details) $ in Thousands		1 Months Ended	3 Months Ended								12 Months Ended
	Nov. 29, 2016 USD ($) shares	Dec. 31, 2016 USD ($) item shares	Dec. 31, 2016 USD ($) item	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
NATURE OF BUSINESS AND OVERVIEW
Number of drug candidates in clinical development | item		7	7								7
Cash, cash equivalents and marketable securities		$ 189,800	$ 189,800								$ 189,800
Net Loss			$ (32,307)	$ (29,598)	$ (31,952)	$ (34,673)	$ (32,684)	$ (31,980)	$ (32,359)	$ (30,174)	(128,530)	$ (127,197)	$ (118,080)
Net cash used in operations											$ 113,036	$ 98,887	$ 101,537
Minimum number of programs delayed or discontinued if unable to raise funds necessary to meet long-term liquidity needs | item											1
Kolltan
NATURE OF BUSINESS AND OVERVIEW
Shares issued as part of consideration (in shares) | shares	18,257,996
Shares issuable in lieu of cash severance obligations (in shares) | shares	437,901
Shares issued in lieu of cash severance obligations (in shares) | shares		111,111
Maximum amount of milestone payments	$ 172,500
Days used for comparable average closing price (in days)	5 days
Ending day of comparable average closing prices (in days)	3 days
Threshold of stock issuance which would require stockholder approval (percentage)	19.9