Organization and Basis of Presentation - Schedule of Retrospective Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating and maintenance expense	$ 330.4	$ 140.6	$ 754.2	$ 415.6
Total operating expenses	1,049.4	529.4	2,292.6	1,419.7
Income from operations	533.1	264.9	783.5	556.5
Other expense	(30.8)	(11.5)	(55.2)	(29.6)
Total other income (expense), net	(24.3)	(6.6)	(43.7)	(20.4)
As Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating and maintenance expense		145.5		430.6
Total operating expenses		534.3		1,434.7
Income from operations		260.0		541.5
Other expense		(6.6)		(14.6)
Total other income (expense), net		(1.7)		(5.4)
Accounting Standards Update 2017-07 [Member] | Effect of Change [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating and maintenance expense		(4.9)		(15.0)
Total operating expenses		(4.9)		(15.0)
Income from operations		4.9		15.0
Other expense		(4.9)		(15.0)
Total other income (expense), net		(4.9)		(15.0)
Westar Energy Inc [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating and maintenance expense	141.7	140.6	491.4	415.6
Total operating expenses	507.9	529.4	1,559.4	1,419.7
Income from operations	256.9	264.9	456.5	556.5
Other expense	(15.2)	(11.5)	(36.1)	(29.6)
Total other income (expense), net	(13.9)	(6.6)	(31.6)	(20.4)
Westar Energy Inc [Member] | As Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating and maintenance expense		145.5		430.6
Total operating expenses		534.3		1,434.7
Income from operations		260.0		541.5
Other expense		(6.6)		(14.6)
Total other income (expense), net		(1.7)		(5.4)
Westar Energy Inc [Member] | Accounting Standards Update 2017-07 [Member] | Effect of Change [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating and maintenance expense		(4.9)		(15.0)
Total operating expenses		(4.9)		(15.0)
Income from operations		4.9		15.0
Other expense		(4.9)		(15.0)
Total other income (expense), net		(4.9)		(15.0)
KCPL [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating and maintenance expense	127.3	114.3	354.5	350.7
Total operating expenses	370.2	375.9	1,043.8	1,058.1
Income from operations	189.4	219.8	365.1	416.2
Other expense	(9.0)	(11.9)	(22.9)	(40.7)
Total other income (expense), net	$ (8.1)	(9.2)	$ (19.1)	(33.8)
KCPL [Member] | As Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating and maintenance expense		124.2		385.0
Total operating expenses		385.8		1,092.4
Income from operations		209.9		381.9
Other expense		(2.0)		(6.4)
Total other income (expense), net		0.7		0.5
KCPL [Member] | Accounting Standards Update 2017-07 [Member] | Effect of Change [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating and maintenance expense		(9.9)		(34.3)
Total operating expenses		(9.9)		(34.3)
Income from operations		9.9		34.3
Other expense		(9.9)		(34.3)
Total other income (expense), net		$ (9.9)		$ (34.3)