INCOME TAXES - Temporary Differences That Give Rise To Accumulated Deferred Income Tax Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets:
Employee Benefits	$ 640.6	$ 637.5
Derivative Liabilities	192.6	172.7
Regulatory Deferrals - Liabilities	290.9	243.5
Allowance for Uncollectible Accounts	76.6	60.5
Tax Effect - Tax Regulatory Liabilities	11.8	9.7
Federal Net Operating Loss Carryforwards	0.0	5.4
Purchase Accounting Adjustment	112.2	119.3
Other	170.5	197.1
Total Deferred Tax Assets	1,495.2	1,445.7
Less: Valuation Allowance	5.1	3.7
Net Deferred Tax Assets	1,490.1	1,442.0
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	5,001.2	4,602.6
Property Tax Accruals	81.9	76.7
Regulatory Amounts:
Regulatory Deferrals - Assets	1,321.8	1,289.1
Tax Effect - Tax Regulatory Assets	252.6	249.3
Goodwill Regulatory Asset - 1999 Merger	186.7	194.9
Derivative Assets	29.5	17.7
Other	223.6	159.4
Total Deferred Tax Liabilities	7,097.3	6,589.7
The Connecticut Light And Power Company [Member]
Deferred Tax Assets:
Employee Benefits	138.8	126.1
Derivative Liabilities	191.5	165.7
Regulatory Deferrals - Liabilities	6.3	36.0
Allowance for Uncollectible Accounts	33.0	30.4
Tax Effect - Tax Regulatory Liabilities	4.9	3.1
Federal Net Operating Loss Carryforwards	0.0	0.0
Other	59.4	55.5
Total Deferred Tax Assets	433.9	416.8
Less: Valuation Allowance	4.5	3.1
Net Deferred Tax Assets	429.4	413.7
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	1,700.3	1,545.6
Property Tax Accruals	29.7	27.3
Regulatory Amounts:
Regulatory Deferrals - Assets	473.4	456.8
Tax Effect - Tax Regulatory Assets	170.4	168.7
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	27.0	17.7
Other	16.3	18.5
Total Deferred Tax Liabilities	2,417.1	2,234.6
NSTAR Electric Company [Member]
Deferred Tax Assets:
Employee Benefits	58.4	91.3
Derivative Liabilities	1.1	0.6
Regulatory Deferrals - Liabilities	186.4	109.4
Allowance for Uncollectible Accounts	20.0	8.5
Tax Effect - Tax Regulatory Liabilities	1.1	1.5
Federal Net Operating Loss Carryforwards	0.0	0.0
Other	2.2	3.4
Total Deferred Tax Assets	269.2	214.7
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	269.2	214.7
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	1,463.5	1,387.1
Property Tax Accruals	25.6	22.8
Regulatory Amounts:
Regulatory Deferrals - Assets	322.3	339.7
Tax Effect - Tax Regulatory Assets	36.1	36.0
Goodwill Regulatory Asset - 1999 Merger	160.3	167.4
Derivative Assets	0.0	0.0
Other	97.7	22.0
Total Deferred Tax Liabilities	2,105.5	1,975.0
Public Service Company Of New Hampshire [Member]
Deferred Tax Assets:
Employee Benefits	46.5	37.1
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	36.7	42.1
Allowance for Uncollectible Accounts	4.1	3.6
Tax Effect - Tax Regulatory Liabilities	2.6	2.3
Federal Net Operating Loss Carryforwards	0.0	2.4
Other	56.4	61.1
Total Deferred Tax Assets	146.3	148.6
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	146.3	148.6
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	726.3	655.3
Property Tax Accruals	8.0	7.3
Regulatory Amounts:
Regulatory Deferrals - Assets	142.1	137.9
Tax Effect - Tax Regulatory Assets	12.2	15.4
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	0.0	0.0
Other	43.1	38.6
Total Deferred Tax Liabilities	931.7	854.5
Western Massachusetts Electric Company [Member]
Deferred Tax Assets:
Employee Benefits	11.1	10.0
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	8.5	6.1
Allowance for Uncollectible Accounts	5.7	4.5
Tax Effect - Tax Regulatory Liabilities	2.2	2.4
Federal Net Operating Loss Carryforwards	0.0	0.4
Other	4.4	5.0
Total Deferred Tax Assets	31.9	28.4
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	31.9	28.4
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	438.4	416.1
Property Tax Accruals	11.2	10.6
Regulatory Amounts:
Regulatory Deferrals - Assets	59.4	60.5
Tax Effect - Tax Regulatory Assets	8.7	9.0
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	0.0	0.0
Other	5.0	2.7
Total Deferred Tax Liabilities	$ 522.7	$ 498.9