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The aggregate revenue, whether regulated or unregulated, derived from the generation, transmission and distribution of electricity.

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Organization and Basis of Presentation - New Accounting Standards (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash flows provided by (used in) operating activities			$ 1,191.6	$ 742.7
Cash flows provided by (used in) investing activities			574.8	(581.4)
Cash, cash equivalents and restricted cash	$ 679.8	$ 3.5	679.8	3.5	$ 3.5	$ 3.2
Operating revenues	(1,582.5)	(794.3)	(3,076.1)	(1,976.2)
Taxes other than income tax	(83.0)	(41.8)	(183.5)	(126.4)
Accounting Standards Update 2016-15 - Corporate Owned Life Insurance, Premiums [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash flows provided by (used in) operating activities				2.3
Cash flows provided by (used in) investing activities				(2.3)
Accounting Standards Update 2016-15 - Corporate Owned Life Insurance, Settlements [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash flows provided by (used in) operating activities				(1.9)
Cash flows provided by (used in) investing activities				1.9
Accounting Standards Update 2016-18 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash, cash equivalents and restricted cash		0.1		0.1		0.1
Westar Energy Inc [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash flows provided by (used in) operating activities			707.2	742.7
Cash flows provided by (used in) investing activities			(523.0)	(581.4)
Cash, cash equivalents and restricted cash	3.4	3.5	3.4	3.5	$ 3.5	3.2
Operating revenues	(764.8)	(794.3)	(2,015.9)	(1,976.2)
Taxes other than income tax	(42.2)	(41.8)	(128.8)	(126.4)
Westar Energy Inc [Member] \| Accounting Standards Update 2016-15 - Corporate Owned Life Insurance, Premiums [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash flows provided by (used in) operating activities				2.3
Cash flows provided by (used in) investing activities				(2.3)
Westar Energy Inc [Member] \| Accounting Standards Update 2016-15 - Corporate Owned Life Insurance, Settlements [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash flows provided by (used in) operating activities				(1.9)
Cash flows provided by (used in) investing activities				1.9
Westar Energy Inc [Member] \| Accounting Standards Update 2016-18 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash, cash equivalents and restricted cash		$ 0.1		$ 0.1		$ 0.1
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] \| Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues	22.1		59.7
Taxes other than income tax	$ 22.1		$ 59.7