Related Party Transactions (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Service Company [Member]
Related Party Transaction [Line Items]
Costs incurred by related party	$ 20.5	$ 18.3
Prepaid balance to related party	4.4		$ 1.2
Director [Member]
Related Party Transaction [Line Items]
Billings from related vendor	190.0	152.9
Accounts payable to related vendor	6.8		$ 34.0
Ipalco Enterprises, Inc. [Member]
Related Party Transaction [Line Items]
Costs incurred by related party	$ 6.5	$ 5.6