This item represents the change in fair value of the Symphony Icon, Inc. purchase liability. In connection with the acquisition of Symphony Icon, Lexicon made an initial estimate of the fair value of the liability for the associated based and contingent payments. Changes in this liability, based on the development of the programs and the time until such payments are expected to be made, are recorded in Lexicon's consolidated statements of operations.

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Arrangements with Symphony Icon Inc (Details) - USD ($) $ in Thousands	1 Months Ended				3 Months Ended
	Mar. 31, 2017	Sep. 30, 2016	Apr. 30, 2015	Dec. 31, 2014	Mar. 31, 2018	Mar. 31, 2017	Jul. 30, 2012	Jul. 30, 2010	Jun. 15, 2007
Loss Contingencies [Line Items]
Holdings Contribution to Icon									$ 45,000
Lexicon Sold Shares to Holdings									1,092,946
Lexicon Received Cash from Holdings									$ 15,000
Lexicon Paid Holdings Cash								$ 10,000
Symphony Base Payment in Shares							1,891,074
Symphony Base Payment Obligation							$ 35,000
Symphony Contingent Payment Maximum								$ 45,000
Symphony Contingent Payment Percentage								50.00%
Symphony Regulatory Approval Payment								$ 15,000
Symphony Regulatory Approval Reduction Percentage								50.00%
Symphony Regulatory Approval Percentage Limit								50.00%
Symphony Payment in Stock Limitation								50.00%
Symphony Contingent Payment In Cash	$ 10,500	$ 3,200	$ 750	$ 5,800
Symphony Amendment Buyout		$ 21,000
Symphony Contingent Payment in Shares	659,905			666,111
Symphony Contingent Payment Total				$ 11,500
Symphony Fair Value of Base and Contingent Payments								$ 45,600
Symphony Base Payment Discount Rate								14.00%
Symphony Contingent Payment Discount Rate								18.00%
Increase in fair value of Symphony Icon, Inc. purchase liability					$ 0	$ 2,101