Collaboration and License Agreements (Details) $ in Thousands, € in Millions	1 Months Ended							3 Months Ended				9 Months Ended	12 Months Ended
	Feb. 28, 2019 USD ($)	Nov. 30, 2017 USD ($)	Oct. 31, 2017 USD ($)	Nov. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Oct. 31, 2014 USD ($)	Oct. 31, 2014 EUR (€)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Collaboration and License Agreements [Abstract]
Sanofi Upfront Payment				$ 300,000
Sanofi Development Milestones				110,000
Sanofi Regulatory Milestones				220,000
Sanofi Outcomes Study Milestone				100,000
Sanofi Sales Milestone Payments				990,000
Sanofi Commercialization Costs Funded by Lexicon Maximum Amount				100,000
Sanofi Revenue Allocated to License Deliverable				126,800
Sanofi Revenue Allocated to Development Deliverable				113,800
Sanofi Revenue Allocated to Funding Deliverable				$ 59,400
Sanofi Revenue Recognized								$ 100		$ 19,300
Ipsen Total Payments								47,200
Ipsen Maximum Regulatory And Commercial Milestones								9,600
Ipsen Maximum Sales Milestones | €							€ 72
Ipsen Total Upfront Payments					$ 24,500
Ipsen Milestone Payment										5,100	$ 6,400	$ 6,400
Ipsen Milestone Payment Received	$ 2,250	$ 3,840	$ 3,840						$ 1,250	5,100
Ipsen Revenue Allocated to License Deliverable													$ 1,400	$ 21,200
Ipsen Revenue Allocated to Development Deliverable						$ 1,700
Ipsen Revenue Allocated to Committee Deliverable						$ 100
Ipsen Revenue Recognized								$ 2,300		$ 300