Collaboration and License Agreements (Details) $ in Thousands, € in Millions	1 Months Ended						3 Months Ended		6 Months Ended		9 Months Ended		12 Months Ended
	Nov. 30, 2017 USD ($)	Oct. 31, 2017 USD ($)	Nov. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Oct. 31, 2014 USD ($)	Oct. 31, 2014 EUR (€)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	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									$ 25,100	$ 24,100
Ipsen Total Payments									43,700
Ipsen Maximum Regulatory And Commercial Milestones									13,000
Ipsen Maximum Sales Milestones | €						€ 72
Ipsen Total Upfront Payments				$ 24,500
Ipsen Milestone Payment								$ 6,400			$ 5,100	$ 6,400
Ipsen Milestone Payment Received	$ 3,840	$ 3,840					$ 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									600	1,400
Ipsen Product Revenue									$ 1,400	$ 200