Organization and Basis of Presentation - Summary of Reclassifications (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
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease in pension and post-retirement liability	$ (949.7)		$ (949.7)		$ (491.2)
Increase in other long-term liabilities	231.9		231.9		133.3
Amortization of deferred refueling outage			14.7	$ 12.1
Deferred income taxes, net	93.9	$ 73.1	50.3	128.7
Investment tax credit amortization	(1.0)	(0.6)	(2.4)	(2.0)
Net deferred income taxes and credits			47.8	127.0
Payments for asset retirement obligations			(15.9)	(1.9)
Other			(1.9)	(5.3)
Fuel inventory and supplies			44.6	15.5
Prepaid expenses and other current assets			(3.4)	55.5
Other current liabilities			38.9	(108.5)
Changes in other assets			26.0	22.9
Changes in other liabilities			33.9	5.5
Cash Flows from Operating Activities			1,191.6	742.7
Additions to property, plant and equipment			(698.3)	(564.6)
Other investing activities			(15.2)	(3.3)
Cash Flows from (used in) Investing Activities			574.8	(581.4)
Westar Energy Inc [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease in pension and post-retirement liability	(465.2)		(465.2)		(491.2)
Increase in other long-term liabilities	127.8		127.8		133.3
Amortization of deferred refueling outage			10.5	12.1
Deferred income taxes, net	24.3	73.1	(32.2)	128.7
Investment tax credit amortization	(0.7)	(0.6)	(2.0)	(2.0)
Net deferred income taxes and credits			(34.2)	127.0
Payments for asset retirement obligations			(10.6)	(1.9)
Other			(1.9)	(5.3)
Fuel inventory and supplies			25.4	15.5
Prepaid expenses and other current assets			(7.0)	55.5
Other current liabilities			23.2	(108.5)
Changes in other assets			22.9	22.9
Changes in other liabilities			(26.7)	5.5
Cash Flows from Operating Activities			707.2	742.7
Additions to property, plant and equipment			(510.3)	(564.6)
Other investing activities			(8.1)	(3.3)
Cash Flows from (used in) Investing Activities			(523.0)	(581.4)
Westar Energy Inc [Member] | Restatement Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease in pension and post-retirement liability					50.2
Increase in other long-term liabilities					(50.2)
KCPL [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Decrease in pension and post-retirement liability	(522.3)		(522.3)		(512.2)
Increase in other long-term liabilities	80.7		80.7		$ 61.6
Amortization of deferred refueling outage			10.3	13.7
Deferred income taxes, net	0.8	13.1	26.0	33.4
Investment tax credit amortization	$ (0.3)	$ (0.3)	(0.8)	(0.8)
Net deferred income taxes and credits			25.2	32.6
Payments for asset retirement obligations			(9.9)	(14.9)
Other			2.7	6.3
Fuel inventory and supplies			13.2	3.6
Prepaid expenses and other current assets			(3.6)	8.9
Other current liabilities			8.5	9.9
Changes in other assets			28.3	52.6
Changes in other liabilities			79.0	7.3
Cash Flows from Operating Activities			504.5	526.6
Additions to property, plant and equipment			(310.6)	(317.1)
Other investing activities			3.4	0.8
Cash Flows from (used in) Investing Activities			$ (312.6)	(318.8)
KCPL [Member] | As Previously Reported [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Amortization of other				23.4
Amortization of deferred refueling outage				0.0
Deferred income taxes, net				33.4
Investment tax credit amortization				(0.8)
Net deferred income taxes and credits				0.0
Payments for asset retirement obligations				0.0
Other				(16.0)
Fuel inventory and supplies				0.0
Fuel inventories				10.8
Materials and supplies				(7.2)
Prepaid expenses and other current assets				0.0
Accrued interest				8.1
Other current liabilities				0.0
Changes in other assets				0.0
Changes in other liabilities				0.0
Deferred refueling outage costs				12.1
Pension and post-retirement benefit obligations				48.6
Fuel recovery mechanisms				7.6
Cash Flows from Operating Activities				120.0
Additions to property, plant and equipment				0.0
Utility capital expenditures				(295.1)
Allowance for borrowed funds used during construction				(4.2)
Other investing activities				(17.0)
Cash Flows from (used in) Investing Activities				(316.3)
KCPL [Member] | Restatement Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Amortization of other				0.0
Amortization of deferred refueling outage				13.7
Deferred income taxes, net				0.0
Investment tax credit amortization				0.0
Net deferred income taxes and credits				32.6
Payments for asset retirement obligations				(14.9)
Other				6.3
Fuel inventory and supplies				3.6
Fuel inventories				0.0
Materials and supplies				0.0
Prepaid expenses and other current assets				8.9
Accrued interest				0.0
Other current liabilities				9.9
Changes in other assets				52.6
Changes in other liabilities				7.3
Deferred refueling outage costs				0.0
Pension and post-retirement benefit obligations				0.0
Fuel recovery mechanisms				0.0
Cash Flows from Operating Activities				120.0
Additions to property, plant and equipment				(317.1)
Utility capital expenditures				0.0
Allowance for borrowed funds used during construction				0.0
Other investing activities				0.8
Cash Flows from (used in) Investing Activities				$ (316.3)