INCOME TAXES - Income Taxes (Details) $ in Millions	3 Months Ended
Jun. 30, 2018 USD ($)
Income Tax Disclosure [Abstract]
Excess accumulated deferred taxes returned	$ 1.9

Regulatory Order, Excess Deferred Income Tax Returned To Customers

No definition available.

Name:	lg_RegulatoryOrderExcessDeferredIncomeTaxReturnedToCustomers
Namespace Prefix:	lg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration