CONTINGENCIES AND COMMITMENTS (Butte Fire) (Details) $ in Millions							1 Months Ended		2 Months Ended	3 Months Ended	9 Months Ended
	Oct. 30, 2018 contractor household complaint plaintiff	Sep. 06, 2018 plaintiff	Mar. 02, 2018 USD ($)	Mar. 01, 2018 USD ($)	Apr. 25, 2017 USD ($) citation	Apr. 13, 2017 USD ($)	Oct. 31, 2018 USD ($)	May 31, 2017 USD ($)	Sep. 30, 2018 USD ($) plaintiff	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	May 23, 2016 contractor	Apr. 28, 2016 a fatality structure outbuilding home comercial_property
Butte Fire
Loss Contingencies [Line Items]
Number of acres burned (acre) | a													70,868
Number of fatalities caused by fire (fatality) | fatality													2
Number of homes destroyed by fire (home) | home													549
Number of outbuildings damaged by fire (outbuilding) | outbuilding													368
Number of commercial properties damaged by fire (commercial property) | comercial_property													4
Number of structures damaged (structure) | structure													44
Number of vegetation management contractors (contractor) | contractor												2
Number of plaintiffs, smaller public entities (plaintiff) | plaintiff									5
Number of plaintiffs, fire districts (plaintiff) | plaintiff									3
Number of plaintiffs, water district (plaintiff) | plaintiff									1
Fire fighting costs recovery requested						$ 87.0
Value of claims brought against the company								$ 190.0
Estimate of additional possible losses									$ 200.0	$ 200.0	$ 200.0
Cumulative legal expenses incurred											118.0
Legal expenses incurred										9.0	31.0
Coverage for third party liability											922.0
Probable insurance recoveries											922.0
Reimbursements from insurance policies											60.0
Reimbursements from insurance policies											7.0
Number of citations (citation) | citation					2
Value of citations issued					$ 8.3
Butte Fire | Minimum
Loss Contingencies [Line Items]
Probable loss to be incurred									1,100.0	1,100.0	1,100.0
Butte Fire | Maximum
Loss Contingencies [Line Items]
Probable loss to be incurred									$ 1,300.0	$ 1,300.0	$ 1,300.0
Butte Fire | Agreement Reached In Litigation To Stipulate To Judgment On Inverse Condemnation Grounds [Member]
Loss Contingencies [Line Items]
Number of plaintiffs in lawsuit (plaintiff) | plaintiff		2
Butte Fire | County of Calaveras
Loss Contingencies [Line Items]
Value of claims brought against company			$ 167.0	$ 85.0
Subsequent Event
Loss Contingencies [Line Items]
Number of vegetation management contractors dismissed from complaints (contractor) | contractor	2
Subsequent Event | Butte Fire
Loss Contingencies [Line Items]
Number of vegetation management contractors (contractor) | contractor	2
Number of complaints filed (complaint) | complaint	95
Number of plaintiffs (plaintiff) | plaintiff	4,000
Number of households represented in court (household) | household	2,100
Reimbursements from insurance policies							$ 45.0