Taxes Tax Reform (Details) - USD ($) $ in Millions	Jun. 01, 2018	Sep. 30, 2018	Dec. 31, 2017
Regulatory liabilities		$ 2,357.1	$ 1,094.0
Revenue Subject to Refund [Member]
Regulatory liabilities		155.1
Westar Energy Inc [Member]
Regulatory liabilities		1,136.9	1,094.0
Westar Energy Inc [Member] | Revenue Subject to Refund [Member]
Regulatory liabilities		66.3
KCPL [Member]
Regulatory liabilities		862.9	$ 770.9
Decrease in net deferred income tax liabilities due to tax reform	$ 46.6
Decrease in net deferred income tax liabilities for revaluation of deferred income tax assets and liabilities in rate base	28.8
Decrease in net deferred income tax liabilities for tax gross-up adjustment for ratemaking purposes due to tax reform	9.9
Income tax benefit from tax reform	$ 15.5
KCPL [Member] | Revenue Subject to Refund [Member]
Regulatory liabilities		$ 64.6