Collaboration and License Agreements (Details) € in Millions, $ in Millions	1 Months Ended				6 Months Ended		12 Months Ended
	Nov. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Oct. 31, 2014 USD ($)	Oct. 31, 2014 EUR (€)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Collaboration and License Agreements [Abstract]
Sanofi Upfront Payment	$ 300.0
Sanofi Development and Regulatory Milestone Payments	430.0
Sanofi Sales Milestone Payments	990.0
Sanofi Commercialization Costs Funded by Lexicon Maximum Amount	100.0
Sanofi Revenue Allocated to License Deliverable	126.8
Sanofi Revenue Allocated to Development Deliverable	113.8
Sanofi Revenue Allocated to Funding Deliverable	$ 59.4
Sanofi Revenue Recognized					$ 31.7
Ipsen Maximum Regulatory And Commercial Milestones			$ 34.0
Ipsen Maximum Sales Milestones | €				€ 72
Ipsen Total Upfront Payments		$ 24.5
Ipsen Revenue Allocated to License Deliverable						$ 1.4	$ 21.2
Ipsen Revenue Allocated to Development Deliverable			1.7
Ipsen Revenue Allocated to Committee Deliverable			$ 0.1
Ipsen Revenue Recognized					$ 0.2	$ 2.0