Related Parties (Service Agreements) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Corporate Service Billings [Member] | IPL [Member]
Related Party Transactions [Line Items]
Amounts billed between related parties	$ 41	$ 36	$ 74	$ 68
Corporate Service Billings [Member] | WPL [Member]
Related Party Transactions [Line Items]
Amounts billed between related parties	31	23	59	49
Sales Credited [Member] | IPL [Member]
Related Party Transactions [Line Items]
Amounts billed between related parties	2	1	4	3
Sales Credited [Member] | WPL [Member]
Related Party Transactions [Line Items]
Amounts billed between related parties	3	4	2	7
Purchases Billed [Member] | IPL [Member]
Related Party Transactions [Line Items]
Amounts billed between related parties	108	76	207	152
Purchases Billed [Member] | WPL [Member]
Related Party Transactions [Line Items]
Amounts billed between related parties	$ 28	$ 14	$ 58	$ 28