Contingencies (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 1997	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2008	Dec. 31, 1993
Loss Contingencies [Line Items]
Malpractice Loss Contingency, Letters of Credit and Surety Bonds	$ 145.6	$ 145.6
Long-term Purchase Commitment, Amount	200.0	200.0
Loss Contingency, Damages Sought, Value		1,200.0
Construction in Progress, Gross	634.0	634.0
Total capital spending on new generation asset	640.0	640.0
Liability for Title Claims and Claims Adjustment Expense	17.0	17.0
Loss Contingency Accrual	0.0	$ 0.0
Site Contingency, Names of Other Potentially Responsible Parties		100	69
Litigation Settlement, Expense		$ 115.0
Loss Contingency, Range of Possible Loss, Maximum	1,700.0	1,700.0
Loss Contingency, Range of Possible Loss, Portion Not Accrued	500.0	500.0
Loss Contingency, Inestimable Loss	1,050.0	1,050.0
investment in Trojan							87.00%
Refund to customers for Trojan Investment including interest						$ 33.0
Class action damages sought					$ 260.0
Litigation Settlement, Amount	$ 1.0	$ 79.0
Jointly Owned Utility Plant, Proportionate Ownership Share of Colstrip	20.00%	20.00%
Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base				$ 514.0