Equity Compensation (Details) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
	Jun. 04, 2018 USD ($) shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2018 USD ($) $ / shares Rate shares	Sep. 30, 2017 USD ($) $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award Accelerated Compensation Cost | $				$ 14.6
Equity compensation expense | $		$ 4.3	$ 2.1	26.0	$ 6.7
Income tax (expense) benefit | $		$ (1.5)	$ 0.8	$ 1.9	$ 2.6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Ending balance (in shares)	204,825
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning balance (in shares)				255,964
Granted (in shares)		0	0	222,465
Converted Great Plains Energy award upon merger (in shares)				82,331
Vested (in shares)		0		(342,599)
Forfeited (in shares)				(905)
Ending balance (in shares)		217,256		217,256
Weighted average beginning balance (in dollars per share) | $ / shares				$ 46.09
Weighted average granted (in dollars per share) | $ / shares				52.16	$ 53.25
Weighted average converted (in dollars per share) | $ / shares				53.77
Weighted average vested (in dollars per share) | $ / shares				46.81
Weighted average forfeited (in dollars per share) | $ / shares				50.73
Weighted average ending balance (in dollars per share) | $ / shares		$ 54.07		$ 54.07
Total unrecognized compensation expense | $		$ 9.5		$ 9.5
Total fair value of shares vested | $			$ 0.2	$ 16.0	$ 3.8
Remaining weighted-average contractual term (in years)				1 year 8 months 15 days
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum (in hundredths) | Rate				16.60%
Expected volatility, maximum (in hundredths) | Rate				18.50%
Expected dividend yield (in hundredths) | Rate				2.96%
Risk-free interest rate, minimum (in hundredths) | Rate				1.80%
Risk-free interest rate, maximum (in hundredths) | Rate				2.60%
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning balance (in shares)				0
Granted (in shares)		0	0
Converted Great Plains Energy award upon merger (in shares)				351,708
Forfeited (in shares)				(3,212)
Ending balance (in shares)		348,496		348,496
Weighted average beginning balance (in dollars per share) | $ / shares				$ 0.00
Weighted average granted (in dollars per share) | $ / shares				63.79
Weighted average converted (in dollars per share) | $ / shares				63.79
Weighted average forfeited (in dollars per share) | $ / shares				63.44
Weighted average ending balance (in dollars per share) | $ / shares		$ 63.80		$ 63.80
Total unrecognized compensation expense | $		$ 10.6		$ 10.6
Remaining weighted-average contractual term (in years)				1 year 3 months 20 days
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning balance (in shares)				0
Granted (in shares)		0	0
Converted Great Plains Energy award upon merger (in shares)				122,505
Vested (in shares)				(4,222)
Forfeited (in shares)				(1,070)
Ending balance (in shares)		117,213		117,213
Weighted average beginning balance (in dollars per share) | $ / shares				$ 0.00
Weighted average granted (in dollars per share) | $ / shares				54.05
Weighted average converted (in dollars per share) | $ / shares				54.05
Weighted average vested (in dollars per share) | $ / shares				54.50
Weighted average forfeited (in dollars per share) | $ / shares				54.04
Weighted average ending balance (in dollars per share) | $ / shares		$ 54.03		$ 54.03
Total unrecognized compensation expense | $		$ 3.1		$ 3.1
Total fair value of shares vested | $		0.2		$ 0.2
Remaining weighted-average contractual term (in years)				1 year 4 months 26 days
Westar Energy Inc [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award Accelerated Compensation Cost | $				$ 14.6
Equity compensation expense | $		2.0	$ 2.1	22.6	6.7
Income tax (expense) benefit | $		(1.0)	0.8	2.1	2.6
KCPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity compensation expense | $		1.6	1.2	4.8	2.9
Income tax (expense) benefit | $		$ (0.7)	$ 0.4	$ (0.1)	$ 1.2
Great Plains Energy [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exchange ratio	0.5981
Stock Compensation Plan [Member] | Great Plains Energy [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of shares issued | $	$ 12.5