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Fair Value of Financial Assets and Liabilities, Financial Impact of Qualifying Cash Flow Hedges (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]      
Accumulated other comprehensive loss related to cash flow hedges at Jan. 1 $ (26.5) $ (22.8) $ (23.8)
After-tax net realized losses on derivative transactions reclassified into earnings 1.2 1.0 1.0
Adoption of ASU. 2018-02 (a) [1] 0.0 (4.7) 0.0
Accumulated other comprehensive loss related to cash flow hedges at Dec. 31 $ (25.3) $ (26.5) $ (22.8)
[1] In 2017, PSCo implemented ASU No. 2018-02 related to TCJA, which resulted in reclassification of certain credit balances within net accumulated other comprehensive loss to retained earnings.