FAIR VALUE MEASUREMENTS - Debt (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notional contract amount			$ 500,000,000.0
Fair value adjustments	$ (19,200,000)		(5,400,000)
Loss on derivative hedging instruments	13,800,000	$ 0
Book Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	7,827,000,000		7,341,700,000
Book Value | Fair Value, Measurements, Recurring | Variable Interest Entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	51,100,000		81,400,000
Fair Value | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	8,130,500,000		7,412,100,000
Fair Value | Fair Value, Measurements, Recurring | Level 2 | Variable Interest Entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	50,900,000		81,300,000
Westar Energy | Book Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	3,690,000,000		3,689,800,000
Westar Energy | Book Value | Fair Value, Measurements, Recurring | Variable Interest Entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	51,100,000		81,400,000
Westar Energy | Fair Value | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	3,884,400,000		3,771,300,000
Westar Energy | Fair Value | Fair Value, Measurements, Recurring | Level 2 | Variable Interest Entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	50,900,000		81,300,000
KCP&L | Book Value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	2,924,600,000		2,530,100,000
KCP&L | Fair Value | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	3,130,200,000		2,637,500,000
Long-Term debt fair value adjustment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt gross	$ 136,800,000		$ 144,800,000