Retirement Benefits - Textuals (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2015
Qualified Pension Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension and Other Postretirement Benefit Plans, Net Prior Service Cost or Credit, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year	$ 3.9
SERP Pension Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year, Net Gain (Loss)	1.6
PUGET SOUND ENERGY, INC.
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent	4.00%	1.00%
PUGET SOUND ENERGY, INC. | Qualified Pension Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year, Net Gain (Loss)	20.5
Pension and Other Postretirement Benefit Plans, Net Prior Service Cost or Credit, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year	1.6
PUGET SOUND ENERGY, INC. | SERP Pension Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year, Net Gain (Loss)	2.1
PUGET SOUND ENERGY, INC. | Other Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year, Net Gain (Loss)	0.3
Cash Balance Formula | PUGET SOUND ENERGY, INC.
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent	100.00%
Final Average Earnings Formula | PUGET SOUND ENERGY, INC.
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent	55.00%
PUGET SOUND ENERGY, INC. | Qualified Pension Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated Future Employer Contributions in Current Fiscal Year			18.0
PUGET SOUND ENERGY, INC. | SERP Pension Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated Future Employer Contributions in Current Fiscal Year			4.4
PUGET SOUND ENERGY, INC. | Other Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated Future Employer Contributions in Current Fiscal Year			$ 0.5