Receivables (Receivables Sold Under The Agreement) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Receivables [Line Items]
Fair value of deferred proceeds	$ 243.7	$ 222.1
IPL [Member]
Receivables [Line Items]
Fair value of deferred proceeds	243.7	222.1
Receivables Sold [Member] | IPL [Member]
Receivables [Line Items]
Customer accounts receivable	179.0	133.8
Unbilled utility revenues	78.2	112.7
Other receivables	0.1	0.3
Receivables sold to third party	257.3	246.8
Less: cash proceeds	1.0	12.0
Deferred proceeds	256.3	234.8
Less: allowance for doubtful accounts	12.6	12.7
Fair value of deferred proceeds	$ 243.7	$ 222.1