SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Standards (Details) $ in Millions	Mar. 31, 2017 USD ($)
Accounting Policies [Abstract]
Available for sale securities subject to new accounting guidance	$ 49

Amount represents the fair value of the available for sale equity securities that will be subject to the new accounting guidance requiring changes in fair value to be recorded in income

No definition available.

Name:	es_AvailableForSaleSecuritiesSubjectToNewAccountingGuidance
Namespace Prefix:	es_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant