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ASSET RETIREMENT OBLIGATIONS (Tables)
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Change in Asset Retirement Obligation
A reconciliation of the beginning and ending carrying amounts of ARO liabilities is as follows:
 
As of December 31,
 
2018
 
2017
(Millions of Dollars)
Eversource
 
CL&P
 
NSTAR
Electric
 
PSNH
 
Eversource
 
CL&P
 
NSTAR
Electric
 
PSNH
Balance as of Beginning of Year
$
419.1

 
$
31.5

 
$
44.6

 
$
25.0

 
$
426.4

 
$
36.0

 
$
42.6

 
$
23.5

Liabilities Incurred During the Year
11.3

 
—

 
11.3

 
—

 
0.2

 
0.1

 
0.1

 
—

Liabilities Settled During the Year
(36.6
)
 
—

 
—

 
(21.5
)
 
(19.3
)
 
(1.0
)
 
(0.2
)
 
—

Accretion
25.5

 
2.0

 
2.2

 
0.5

 
26.3

 
2.3

 
2.1

 
1.5

Revisions in Estimated Cash Flows
46.9

 
—

 
14.3

 
—

 
(14.5
)
 
(5.9
)
 
—

 
—

Balance as of End of Year
$
466.2

 
$
33.5

 
$
72.4

 
$
4.0

 
$
419.1

 
$
31.5

 
$
44.6

 
$
25.0