Receivables (Receivables Sold Under The Agreement) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Receivables [Line Items]
Fair value of deferred proceeds	$ 120.9	$ 222.1
IPL [Member]
Receivables [Line Items]
Fair value of deferred proceeds	120.9	222.1
Receivables Sold [Member] | IPL [Member]
Receivables [Line Items]
Customer accounts receivable	156.1	133.8
Unbilled utility revenues	93.2	112.7
Other receivables	0.3	0.3
Receivables sold to third party	249.6	246.8
Less: cash proceeds	116.0	12.0
Deferred proceeds	133.6	234.8
Less: allowance for doubtful accounts	12.7	12.7
Fair value of deferred proceeds	$ 120.9	$ 222.1