SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair value measurements
Cash and cash equivalents	$ 41,135	$ 58,131	$ 12,851	$ 13,345
Marketable securities	265,744	224,625
Fair Value Inputs Level 1
Fair value measurements
Total	37,435	53,569
Fair Value Inputs Level 1 | Cash and Money Market
Fair value measurements
Cash and cash equivalents	37,435	53,569
Fair Value Inputs Level 2
Fair value measurements
Total	269,444	229,187
Fair Value Inputs Level 2 | Corporate bonds
Fair value measurements
Cash and cash equivalents		4,562
Marketable securities	179,211	141,124
Fair Value Inputs Level 2 | U.S. government agency securities
Fair value measurements
Marketable securities	7,039	18,688
Fair Value Inputs Level 2 | Certificates of deposit
Fair value measurements
Marketable securities	1,999
Fair Value Inputs Level 2 | Commercial Paper.
Fair value measurements
Cash and cash equivalents	3,700
Marketable securities		3,993
Fair Value Inputs Level 2 | Asset backed securities
Fair value measurements
Marketable securities	77,495	60,820
Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Total	306,879	282,756
Estimate Of Fair Value Fair Value Disclosure | Cash and Money Market
Fair value measurements
Cash and cash equivalents	37,435	53,569
Estimate Of Fair Value Fair Value Disclosure | Corporate bonds
Fair value measurements
Cash and cash equivalents		4,562
Marketable securities	179,211	141,124
Estimate Of Fair Value Fair Value Disclosure | U.S. government agency securities
Fair value measurements
Marketable securities	7,039	18,688
Estimate Of Fair Value Fair Value Disclosure | Certificates of deposit
Fair value measurements
Marketable securities	1,999
Estimate Of Fair Value Fair Value Disclosure | Commercial Paper.
Fair value measurements
Cash and cash equivalents	3,700
Marketable securities		3,993
Estimate Of Fair Value Fair Value Disclosure | Asset backed securities
Fair value measurements
Marketable securities	$ 77,495	$ 60,820