Fair Value Measurements (Details) $ in Millions	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Assets [Abstract]
Derivative instruments, assets	$ 5.3	$ 3.3
Liabilities [Abstract]
Derivative instruments, liabilities	80.8	6.4
Kansas City Power and Light Company [Member]
Assets [Abstract]
Derivative instruments, assets	1.1	0.2
Liabilities [Abstract]
Derivative instruments, liabilities	0.7	0.7
Other Consolidated Entities [Member] | Fixed Income Funds [Member]
SERP rabbi trust [Abstract]
Alternative Investments, Fair Value Disclosure	16.7	16.6
Fair Value, Measurements, Recurring [Member]
Assets [Abstract]
Derivative instruments, assets	5.3	3.3
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	218.3	200.7
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	13.7	13.6
SERP rabbi trust [Abstract]
SERP Rabbi Trusts, Fair Value Disclosure	0.1	0.1
Total assets	237.4	217.7
Liabilities [Abstract]
Derivative instruments, liabilities	80.8	6.4
Total liabilities	80.8	6.4
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets [Abstract]
Derivative instruments, assets	1.1	0.0
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	178.7	165.4
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	8.9	6.3
SERP rabbi trust [Abstract]
SERP Rabbi Trusts, Fair Value Disclosure	0.1	0.1
Total assets	188.8	171.8
Liabilities [Abstract]
Derivative instruments, liabilities	1.1	5.7
Total liabilities	1.1	5.7
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets [Abstract]
Derivative instruments, assets	2.3	2.7
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	39.6	35.3
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	4.8	7.3
SERP rabbi trust [Abstract]
SERP Rabbi Trusts, Fair Value Disclosure	0.0	0.0
Total assets	46.7	45.3
Liabilities [Abstract]
Derivative instruments, liabilities	0.0	0.0
Total liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets [Abstract]
Derivative instruments, assets	1.9	0.6
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
SERP rabbi trust [Abstract]
SERP Rabbi Trusts, Fair Value Disclosure	0.0	0.0
Total assets	1.9	0.6
Liabilities [Abstract]
Derivative instruments, liabilities	79.7	0.7
Total liabilities	79.7	0.7
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member]
Assets [Abstract]
Derivative instruments, assets	1.1	0.2
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	218.3	200.7
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	13.7	13.6
SERP rabbi trust [Abstract]
Total assets	233.1	214.5
Liabilities [Abstract]
Derivative instruments, liabilities	0.7	0.7
Total liabilities	0.7	0.7
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Equity Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	147.4	135.4
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.9	1.1
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Debt Securities [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	4.8	7.3
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | US Treasury and Government [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	29.7	26.4
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | US Government Corporations and Agencies Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	1.8	1.8
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | US States and Political Subdivisions Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	3.1	4.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Corporate Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	34.4	29.2
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Foreign Government Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.3	0.3
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Cash Equivalents [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	1.6	3.6
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Cash and Cash Equivalents [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	8.0	5.2
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member]
Assets [Abstract]
Derivative instruments, assets	0.0	0.0
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	178.7	165.4
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	8.9	6.3
SERP rabbi trust [Abstract]
Total assets	187.6	171.7
Liabilities [Abstract]
Derivative instruments, liabilities	0.0	0.1
Total liabilities	0.0	0.1
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	147.4	135.4
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.9	1.1
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury and Government [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	29.7	26.4
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | US Government Corporations and Agencies Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | US States and Political Subdivisions Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign Government Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	1.6	3.6
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	8.0	5.2
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member]
Assets [Abstract]
Derivative instruments, assets	0.0	0.0
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	39.6	35.3
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	4.8	7.3
SERP rabbi trust [Abstract]
Total assets	44.4	42.6
Liabilities [Abstract]
Derivative instruments, liabilities	0.0	0.0
Total liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	4.8	7.3
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury and Government [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | US Government Corporations and Agencies Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	1.8	1.8
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	3.1	4.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	34.4	29.2
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Government Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.3	0.3
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Cash Equivalents [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Cash and Cash Equivalents [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member]
Assets [Abstract]
Derivative instruments, assets	1.1	0.2
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
SERP rabbi trust [Abstract]
Total assets	1.1	0.2
Liabilities [Abstract]
Derivative instruments, liabilities	0.7	0.6
Total liabilities	0.7	0.6
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | US Treasury and Government [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | US Government Corporations and Agencies Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | US States and Political Subdivisions Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign Government Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Cash Equivalents [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Cash and Cash Equivalents [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member]
Assets [Abstract]
Derivative instruments, assets	4.2	3.1
SERP rabbi trust [Abstract]
SERP Rabbi Trusts, Fair Value Disclosure	0.1	0.1
Total assets	4.3	3.2
Liabilities [Abstract]
Derivative instruments, liabilities	80.1	5.7
Total liabilities	80.1	5.7
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Commodity Contract [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	1.3
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Interest Rate Contract [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	$ 78.8
Fair Value Inputs, Contingency Factor	0.35
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets [Abstract]
Derivative instruments, assets	$ 1.1	0.0
SERP rabbi trust [Abstract]
SERP Rabbi Trusts, Fair Value Disclosure	0.1	0.1
Total assets	1.2	0.1
Liabilities [Abstract]
Derivative instruments, liabilities	1.1	5.6
Total liabilities	1.1	5.6
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 1 [Member] | Commodity Contract [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	1.1
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 1 [Member] | Interest Rate Contract [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets [Abstract]
Derivative instruments, assets	2.3	2.7
SERP rabbi trust [Abstract]
SERP Rabbi Trusts, Fair Value Disclosure	0.0	0.0
Total assets	2.3	2.7
Liabilities [Abstract]
Derivative instruments, liabilities	0.0	0.0
Total liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 2 [Member] | Commodity Contract [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 2 [Member] | Interest Rate Contract [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets [Abstract]
Derivative instruments, assets	0.8	0.4
SERP rabbi trust [Abstract]
SERP Rabbi Trusts, Fair Value Disclosure	0.0	0.0
Total assets	0.8	0.4
Liabilities [Abstract]
Derivative instruments, liabilities	79.0	0.1
Total liabilities	79.0	$ 0.1
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	0.2
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 3 [Member] | Interest Rate Contract [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	$ 78.8