Collaboration and License Agreements (Details) € in Millions	1 Months Ended							3 Months Ended	12 Months Ended
	Nov. 30, 2017 USD ($)	Oct. 31, 2017 USD ($)	Aug. 31, 2016 USD ($)	Nov. 30, 2015 USD ($)	Oct. 31, 2014 USD ($)	Oct. 31, 2014 EUR (€)	Jul. 31, 2005 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Collaboration and License Agreements [Abstract]
Sanofi Upfront Payment				$ 300,000,000
Sanofi Development Milestones				110,000,000
Sanofi Regulatory Milestones				220,000,000
Sanofi Outcomes Study Milestone				100,000,000
Sanofi Sales Milestone Payments				990,000,000
Sanofi Development Costs Funded by Lexicon Maximum Amount				100,000,000
Sanofi Revenue Allocated to Development Deliverable				113,800,000
Sanofi Revenue Allocated to License Deliverable				126,800,000
Sanofi Revenue Recognized									$ 56,300,000	$ 75,400,000	$ 126,800,000
Sanofi Sales of Clinical Trial Materials									1,900,000	6,300,000
Sanofi Revenue Allocated to Funding Deliverable				$ 59,400,000
Ipsen Total Payments To Date									43,700,000
Ipsen Total Upfront Payments					$ 24,500,000
Ipsen Milestone Payment Received	$ 3,840,000	$ 3,840,000	$ 6,400,000					$ 5,100,000
Ipsen Revenue Allocated to License Deliverable					21,200,000					1,400,000
Ipsen Maximum Regulatory And Commercial Milestones					13,100,000
Ipsen Revenue Allocated to Development Deliverable					1,700,000
Ipsen Revenue Allocated to Committee Deliverable					$ 100,000
Ipsen Revenue Recognized									16,100,000	$ 7,200,000	$ 2,300,000
Ipsen product sales									800,000
Ipsen Maximum Sales Milestones | €						€ 72
TIGM Maximum Exposure									14,200,000
TX Enterprise Fund Award							$ 35,000,000
TX Enterprise Fund Award to Texas AM University System							$ 15,000,000
TIGM Per Job Payment Amount									$ 2,415