Debt (Long-term Debt) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Long-term debt issued		$ 3,870.5	$ 3,802.8
Current maturities		(313.4)	(183.0)
Unamortized debt issuance costs		(22.3)	(22.4)
Unamortized debt (discount) and premium, net		(12.6)	(13.1)
Long-term debt, net		3,522.2	3,584.3
Proceeds from issuance of long-term debt		250.7	812.9	$ 250.0
3.4% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Proceeds from issuance of long-term debt		250.0
Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt issued		1,875.0	1,775.0
Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 100.0	$ 100.0
Interest rate		5.875%	5.875%
Senior Debentures [Member] | 7.25% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		7.25%	7.25%
Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 200.0	$ 200.0
Interest rate		3.65%	3.65%
Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		3.25%	3.25%
Senior Debentures [Member] | 3.4% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt issued	[1]	$ 250.0
Interest rate		3.40%
Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 50.0	$ 50.0
Interest rate		5.50%	5.50%
Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 100.0	$ 100.0
Interest rate		6.45%	6.45%
Senior Debentures [Member] | 6.3% senior debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 125.0	$ 125.0
Interest rate		6.30%	6.30%
Senior Debentures [Member] | 6.25% senior debenture, due 2039 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 300.0	$ 300.0
Interest rate		6.25%	6.25%
Senior Debentures [Member] | 4.7% senior debenture, due 2043 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		4.70%	4.70%
Senior Debentures [Member] | 3.3% senior debenture, retired in 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued			$ 150.0
Interest rate			3.30%
Debentures [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 1,550.0	$ 1,550.0
Debentures [Member] | 5% debenture, due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		5.00%	5.00%
Debentures [Member] | 4.6% debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 150.0	$ 150.0
Interest rate		4.60%	4.60%
Debentures [Member] | 2.25% debenture, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		2.25%	2.25%
Debentures [Member] | 6.25% debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 100.0	$ 100.0
Interest rate		6.25%	6.25%
Debentures [Member] | 6.375% debenture, due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 300.0	$ 300.0
Interest rate		6.375%	6.375%
Debentures [Member] | 7.6% debenture, due 2038 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		7.60%	7.60%
Debentures [Member] | 4.1% debenture, due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		4.10%	4.10%
Pollution Control Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term debt issued			$ 30.6
Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond, retired in 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued			$ 16.0
Interest rate			5.00%
Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bond, retired in 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued			$ 14.6
Interest rate			5.375%
Other Long Term Debt [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 445.5	$ 447.2
Term Loan Credit Agreement [Member] | Term loan credit agreement through 2016, 1% at December 31, 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		1.00%	1.00%
Senior Notes [Member] | 3.45% senior notes, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 75.0	$ 75.0
Interest rate		3.45%	3.45%
Senior Secured Notes [Member] | 5.06% senior secured notes, due 2016 to 2024 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 56.8	$ 58.9
Interest rate		5.06%	5.06%
Other [Member] | Other, 1% at December 31, 2015, due 2016 to 2025 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 3.7	$ 3.3
Interest rate		1.00%	1.00%
IPL [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 1,875.0	$ 1,775.0
Current maturities		0.0	(150.0)
Unamortized debt issuance costs		(11.8)	(10.7)
Unamortized debt (discount) and premium, net		(6.3)	(6.3)
Long-term debt, net		1,856.9	1,608.0
Proceeds from issuance of long-term debt		250.0	250.0	250.0
Repayments of commercial paper		111.0
IPL [Member] | Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt issued		1,875.0	1,775.0
IPL [Member] | Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 100.0	$ 100.0
Interest rate		5.875%	5.875%
IPL [Member] | Senior Debentures [Member] | 7.25% senior debenture, due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		7.25%	7.25%
IPL [Member] | Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 200.0	$ 200.0
Interest rate		3.65%	3.65%
IPL [Member] | Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		3.25%	3.25%
IPL [Member] | Senior Debentures [Member] | 3.4% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt issued	[1]	$ 250.0
Interest rate		3.40%
IPL [Member] | Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 50.0	$ 50.0
Interest rate		5.50%	5.50%
IPL [Member] | Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 100.0	$ 100.0
Interest rate		6.45%	6.45%
IPL [Member] | Senior Debentures [Member] | 6.3% senior debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 125.0	$ 125.0
Interest rate		6.30%	6.30%
IPL [Member] | Senior Debentures [Member] | 6.25% senior debenture, due 2039 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 300.0	$ 300.0
Interest rate		6.25%	6.25%
IPL [Member] | Senior Debentures [Member] | 4.7% senior debenture, due 2043 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		4.70%	4.70%
IPL [Member] | Senior Debentures [Member] | 3.3% senior debenture, retired in 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued			$ 150.0
Interest rate			3.30%
WPL [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 1,550.0	$ 1,580.6
Current maturities		0.0	(30.6)
Unamortized debt issuance costs		(9.9)	(10.8)
Unamortized debt (discount) and premium, net		(6.2)	(6.7)
Long-term debt, net		1,533.9	1,532.5
Proceeds from issuance of long-term debt		0.0	250.0	$ 0.0
WPL [Member] | Debentures [Member]
Debt Instrument [Line Items]
Long-term debt issued		1,550.0	1,550.0
WPL [Member] | Debentures [Member] | 5% debenture, due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		5.00%	5.00%
WPL [Member] | Debentures [Member] | 4.6% debenture, due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 150.0	$ 150.0
Interest rate		4.60%	4.60%
WPL [Member] | Debentures [Member] | 2.25% debenture, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		2.25%	2.25%
WPL [Member] | Debentures [Member] | 6.25% debenture, due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 100.0	$ 100.0
Interest rate		6.25%	6.25%
WPL [Member] | Debentures [Member] | 6.375% debenture, due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 300.0	$ 300.0
Interest rate		6.375%	6.375%
WPL [Member] | Debentures [Member] | 7.6% debenture, due 2038 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		7.60%	7.60%
WPL [Member] | Debentures [Member] | 4.1% debenture, due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 250.0	$ 250.0
Interest rate		4.10%	4.10%
WPL [Member] | Pollution Control Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term debt issued			$ 30.6
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond, retired in 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued			$ 16.0
Interest rate			5.00%
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bond, retired in 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued			$ 14.6
Interest rate			5.375%
Franklin County Holdings [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through 2016, 1% at December 31, 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued		$ 60.0	$ 60.0
Interest rate		1.00%	1.00%

[1] In 2015, IPL issued $250.0 million of 3.4% senior debentures due 2025. The proceeds from the issuance were used by IPL to reduce commercial paper classified as long-term debt by $111 million, reduce cash amounts received from its sales of accounts receivable program and for general corporate purposes.