Merger of Great Plains Energy and Westar Energy - Schedule Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 04, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 2,333.5		$ 0.0
Great Plains Energy [Member]
Business Acquisition [Line Items]
Current assets		$ 2,151.7
Property, plant and equipment, net		9,179.7
Goodwill		2,333.5
Other long-term assets, excluding goodwill		1,235.9
Total assets		14,900.8
Current liabilities		1,673.9
Long-term liabilities, excluding long-term debt		2,892.6
Long-term debt, net		3,358.6
Total liabilities		7,925.1
Total purchase price		$ 6,975.7