SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Standards (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015
Accounting Policies [Abstract]
Available for sale securities subject to new accounting guidance	$ 50.0
Impact of share based compensation accounting guidance on income tax	2.9
Impact of new accounting standard on cash flow statement	$ 9.1	$ 9.7

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.

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