Collaboration and License Agreements (Details) $ in Thousands, € in Millions	1 Months Ended									3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($)	Feb. 28, 2019 USD ($)	Oct. 31, 2018 USD ($)	Nov. 30, 2017 USD ($)	Oct. 31, 2017 USD ($)	Aug. 31, 2016 USD ($)	Nov. 30, 2015 USD ($) milestone	Oct. 31, 2014 USD ($)	Oct. 31, 2014 EUR (€)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collaboration and License Agreements [Abstract]
Sanofi Upfront Payment							$ 300,000
Sanofi Development Milestones							$ 110,000
Number of development milestones | milestone							4
Sanofi Regulatory Milestones							$ 220,000
Number of regulatory milestones | milestone							4
Sanofi Outcomes Study Milestone							$ 100,000
Sanofi Sales Milestone Payments							$ 990,000
Number of commercial milestones | milestone							6
Royalties percentage							40.00%
Funded commercialization costs							40.00%
Development costs period							3 years
Sanofi Development Costs Funded by Lexicon Maximum Amount							$ 100,000
Expiration period							10 years
Sanofi Revenue Allocated to Development Deliverable							$ 113,800
Sanofi Revenue Allocated to License Deliverable							126,800
Sanofi Revenue Recognized												$ 286,000	$ 33,200
Sanofi Revenue Allocated to Funding Deliverable							$ 59,400
Ipsen Total Payments To Date											$ 47,200
Ipsen Total Upfront Payments								$ 24,500
Ipsen Milestone Payment Received		$ 2,300	$ 1,300	$ 3,800	$ 3,800	$ 6,400				$ 5,100
Ipsen Revenue Allocated to License Deliverable								21,200				1,400
Ipsen Maximum Regulatory And Commercial Milestones								9,600
Ipsen Revenue Allocated to Development Deliverable								1,700
Ipsen Revenue Allocated to Committee Deliverable								$ 100
Ipsen Revenue Recognized											300	4,900	$ 4,600
Ipsen product sales											1,300	$ 1,600
Ipsen Maximum Sales Milestones | €									€ 72
Settlement Payment from Termination											208,000
Sanofi Termination Agreement											260,000
Sanofi Deferred Revenue from Termination											23,500
Subsequent Event [Line Items]
Settlement Payment from Termination											$ 208,000
Subsequent Event
Collaboration and License Agreements [Abstract]
Settlement Payment from Termination	$ 26,000
Subsequent Event [Line Items]
Settlement Payment from Termination	$ 26,000