Collaboration and License Agreements (Details) $ in Thousands, € in Millions	1 Months Ended								3 Months Ended	12 Months Ended
	Mar. 31, 2020 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 ($)	Oct. 31, 2014 EUR (€)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 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	$ 60,100
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		$ 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										4,900	4,600	$ 16,200
Ipsen Royalty Income_LXRX										300	100
Ipsen product sales										1,600	$ 800
Ipsen Maximum Sales Milestones | €								€ 72
Settlement Payment from Termination										208,000
Settlement Payment Obligation										26,000
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