Variable Interest Entities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entities (Textuals) [Abstract]
Redeemable Noncontrolling Interest		$ 70.0	$ 0.0
Trent and Desert Sky Wind Farms [Member]
Variable Interest Entities (Textuals) [Abstract]
Non-Affiliate Interest		20.10%
AEP Ownership in LLCs		79.90%
Desert Sky and Trent PPA Period	12 years
Redeemable Noncontrolling Interest		$ 70.0
Current Assets [Member] | Trent and Desert Sky Wind Farms [Member]
Assets [Abstract]
Assets		89.2
Net Property Plant And Equipment [Member] | Trent and Desert Sky Wind Farms [Member]
Assets [Abstract]
Assets		350.8
Net Property Plant And Equipment [Member] | Non-Affiliate Contribution [Member]
Assets [Abstract]
Assets		84.0
Other Noncurrent Assets [Member] | Trent and Desert Sky Wind Farms [Member]
Assets [Abstract]
Assets		0.4
Total Assets [Member] | Trent and Desert Sky Wind Farms [Member]
Assets [Abstract]
Assets		440.4
Current Liabilities [Member] | Trent and Desert Sky Wind Farms [Member]
Liabilities and Equity [Abstract]
Variable Interest Carrying Amount Liabilities and Equity		93.4
Noncurrent Liabilities [Member] | Trent and Desert Sky Wind Farms [Member]
Liabilities and Equity [Abstract]
Variable Interest Carrying Amount Liabilities and Equity		6.1
Equity [Member] | Trent and Desert Sky Wind Farms [Member]
Liabilities and Equity [Abstract]
Variable Interest Carrying Amount Liabilities and Equity		340.9
Total Liabilities And Equity [Member] | Trent and Desert Sky Wind Farms [Member]
Liabilities and Equity [Abstract]
Variable Interest Carrying Amount Liabilities and Equity		$ 440.4