LEASES - Future Minimum Rental Payments, Operating and Capital Leases (Details) $ in Millions	Dec. 31, 2016 USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017	$ 14.1
2018	10.6
2019	8.7
2020	7.0
2021	6.0
Thereafter	10.4
Future minimum lease payments	56.8
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017	2.3
2018	2.3
2019	2.2
2020	2.2
2021	1.7
Thereafter	1.1
Future minimum lease payments	11.8
Less amount representing interest	2.9
Present value of future minimum lease payments	8.9
The Connecticut Light And Power Company [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017	2.0
2018	1.3
2019	1.0
2020	0.7
2021	0.6
Thereafter	1.4
Future minimum lease payments	7.0
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017	1.9
2018	2.0
2019	2.0
2020	2.0
2021	1.4
Thereafter	0.0
Future minimum lease payments	9.3
Less amount representing interest	2.5
Present value of future minimum lease payments	6.8
NSTAR Electric Company [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017	9.0
2018	7.0
2019	5.8
2020	4.8
2021	4.2
Thereafter	6.7
Future minimum lease payments	37.5
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017	0.2
2018	0.2
2019	0.2
2020	0.2
2021	0.3
Thereafter	1.1
Future minimum lease payments	2.2
Less amount representing interest	0.4
Present value of future minimum lease payments	1.8
Public Service Company Of New Hampshire [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017	0.9
2018	0.6
2019	0.5
2020	0.4
2021	0.3
Thereafter	0.8
Future minimum lease payments	3.5
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017	0.2
2018	0.1
2019	0.0
2020	0.0
2021	0.0
Thereafter	0.0
Future minimum lease payments	0.3
Less amount representing interest	0.0
Present value of future minimum lease payments	0.3
Western Massachusetts Electric Company [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017	0.5
2018	0.3
2019	0.3
2020	0.2
2021	0.2
Thereafter	0.4
Future minimum lease payments	$ 1.9

Amount of minimum lease payments for capital leases.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

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Amount of minimum lease payments for capital leases due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of minimum lease payments for capital leases due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of minimum lease payments for capital leases due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of minimum lease payments for capital leases due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of minimum lease payments for capital leases due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of minimum lease payments for capital leases due after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount necessary to reduce net minimum lease payments to present value for capital leases.

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Present value of minimum lease payments for capital leases net of executory costs, including amounts paid by the lessee to the lessor for insurance, maintenance and taxes.

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Amount of required minimum rental payments for leases having an initial or remaining non-cancelable letter-terms in excess of one year.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 55
-Paragraph 40
-Subparagraph (Note 3)
-URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 55
-Paragraph 40
-Subparagraph (Note 3)
-URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697

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