Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 1,040.3		$ 1,060.7		$ 911.9		$ 1,073.3		$ 1,000.8		$ 1,030.3		$ 930.9		$ 1,080.5		$ 4,086.2	$ 4,042.5	$ 4,047.8
Operating income	119.5	[1]	276.9	[1]	189.3	[1]	206.9	[1]	155.3	[1]	326.5	[1]	193.3	[1]	212.9	[1]	792.6	888.0	892.3
Net income	$ 88.6		$ 207.1		$ 122.3		$ 133.7		$ 95.9		$ 186.1		$ 100.6		$ 111.5		$ 551.7	$ 494.1	$ 463.5

[1] (a) In 2018, PSCo implemented ASU No. 2017-07 related to net periodic benefit cost, which resulted in retrospective reclassification of pension costs from O&M expense to other income.