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Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

Genzyme Co-promotion Agreement (Details) (USD $)	9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended						1 Months Ended	9 Months Ended	12 Months Ended
Genzyme Co-promotion Agreement (Details) (USD $)	Sep. 30, 2013	Sep. 30, 2012	Jan. 31, 2012 Co-promotion agreement item	Sep. 30, 2013 Co-promotion agreement	Sep. 30, 2012 Co-promotion agreement	Sep. 30, 2013 Co-promotion agreement item	Sep. 30, 2012 Co-promotion agreement	Dec. 31, 2012 Co-promotion agreement	Sep. 30, 2013 Co-promotion agreement Accounts payable	Sep. 30, 2013 Co-promotion agreement Accrued liabilities	Dec. 31, 2012 Co-promotion agreement Accrued liabilities	Jan. 31, 2013 Co-promotion agreement Genzyme	Sep. 30, 2013 Co-promotion agreement Genzyme	Dec. 31, 2012 Co-promotion agreement Genzyme
Genzyme Co-promotion Agreement
Number of countries outside United States in which marketing authorization is obtained			40
Co-promotion fee received from Genzyme		$ 10,000,000	$ 10,000,000
Additional payments that may be received						3,000,000
Additional payments that may be received for each country outside of the United States in which the company obtains marketing authorization and achieves a specified level of reimbursement						600,000
Number of countries outside United States for which additional payments may be received						5
Percentage of cash receipts of co-promotion fees received by co-promoter												40.00%		50.00%
Percentage of cash receipts of co-promotion fees received by co-promoter for remaining periods													32.00%
Maximum amount to be spent by co-promoter for qualifying clinical development activities in countries that require additional testing													500,000
Prior notice period for termination of agreement						6 months
Percentage of co-promotion fee required to be repaid if agreement terminated prior to January 2014													50.00%
Percentage of co-promotion fee required to be repaid if agreement terminated between January 2014 and January 2015													40.00%
Percentage of co-promotion fee required to be repaid if agreement terminated between January 2015 and January 2016													30.00%
Amortization period of co-promotion fee						4 years
Amortization of co-promotion fee	1,875,000	1,761,000		625,000	625,000	1,900,000	1,800,000
Unamortized balance of the co-promotion fee				5,700,000		5,700,000
Co-promotion expenses				2,300,000	1,600,000	6,000,000	3,300,000
Outstanding obligation to Genzyme				$ 6,000,000		$ 6,000,000		$ 2,200,000	$ 3,700,000	$ 2,300,000	$ 2,200,000