SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Natural Gas And Propane Gas (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Accounting Policies [Abstract]
Difference between replacement cost of gas storage and inventory basis amount	$ 14.5	$ 20.8

The amount by which the replacement cost exceeds (is less than) the cost basis value of stored gas inventories.

No definition available.

Name:	lg_DifferenceBetweenReplacementCostOfGasStorageAndInventoryBasisAmount
Namespace Prefix:	lg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration