SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition, Inventory and Advertising Costs (Details) - USD ($)		3 Months Ended										12 Months Ended
	Jan. 27, 2009	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2010	Mar. 31, 2010	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Aug. 02, 2016	Jul. 24, 2015	Dec. 16, 2014	Apr. 02, 2014
Revenue Recognition
Revenue recognized on an accrual basis												$ 87,037,000	$ 36,364,000	$ 1,388,000
Revenue recognized when cash is received												12,339,000	3,073,000	116,000
Total		$ 35,241,000	$ 28,115,000	$ 21,185,000	$ 14,835,000	$ 14,420,000	$ 12,632,000	$ 8,119,000	$ 4,266,000			$ 99,376,000	$ 39,437,000	1,504,000
Sale of common stock (in shares)		110,236,127				96,674,786						110,236,127	96,674,786
Price of common stock (in dollars per share)		$ 13.36										$ 13.36			$ 15.50	$ 25.50	$ 25.75	$ 12.75
License fee revenue														294,000
Inventory
Raw materials		$ 2,408,000				$ 1,772,000						$ 2,408,000	$ 1,772,000
Semi-finished and finished goods		4,425,000				4,905,000						4,425,000	4,905,000
Total inventory		$ 6,833,000				$ 6,677,000						6,833,000	6,677,000
Advertising Costs
Advertising expense												$ 38,100,000	$ 10,800,000	5,300,000
Collaboration License And Purchase Agreement | Genzyme Corporation
Revenue Recognition
Total agreed consideration amount	$ 18,500,000
Amount of Deferred Revenue	16,650,000
Amount subject to holdback	$ 1,850,000
Initial collaboration period	5 years
Amount received	$ 16,650,000									$ 934,000	$ 962,000
Sale of common stock (in shares)	3,000,000
Price at which share of common stock are sold (in dollars per share)	$ 2.00
Premium above closing price of common stock at which shares are sold (in dollars per share)	0.51
Price of common stock (in dollars per share)	$ 1.49
Aggregate premium received over the closing price of common stock	$ 1,530,000
Amount of premium being amortized	$ 1,530,000
License fee revenue														$ 300,000