Debt (Long-term Debt) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
Long-term debt issued	$ 3,802.8		$ 3,348.4
Current maturities	(183.0)		(358.5)
Unamortized debt (discount) and premium, net	(13.1)		(12.1)
Long-term debt, net	3,606.7		2,977.8
Payments to retire long-term debt	(358.5)		(1.5)		(1.4)
Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt issued	1,775.0		1,525.0
Senior Debentures [Member] | 3.3% senior debenture, due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued	150.0		150.0
Interest rate	3.30%		3.30%
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	[1]	0	[1]
Interest rate	3.25%
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%
Debentures [Member]
Debt Instrument [Line Items]
Long-term debt issued	1,550.0		1,300.0
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	[2]	0	[2]
Interest rate	4.10%
Pollution Control Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term debt issued	30.6		77.5
Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond, due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued	16.0	[3]	24.5	[3]
Interest rate	5.00%		5.00%
Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bond, due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued	14.6		14.6
Interest rate	5.375%		5.375%
Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond [Member]
Debt Instrument [Line Items]
Long-term debt issued	0	[4]	38.4	[4]
Interest rate			5.00%
Other Long Term Debt [Member]
Debt Instrument [Line Items]
Long-term debt issued	447.2		445.9
Term Loan Credit Agreement [Member] | Term loan credit agreement through 2016, 1% at December 31, 2014 [Member]
Debt Instrument [Line Items]
Long-term debt issued	250.0	[5]	0	[5]
Interest rate	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 Notes [Member] | 4% senior notes, due 2014 [Member]
Debt Instrument [Line Items]
Long-term debt issued	0	[5]	250.0	[5]
Interest rate			4.00%
Senior Secured Notes [Member] | 5.06% senior secured notes, due 2015 to 2024 [Member]
Debt Instrument [Line Items]
Long-term debt issued	58.9		60.5
Interest rate	5.06%		5.06%
Other [Member] | Other, 1% at December 31, 2014, due 2015 to 2025 [Member]
Debt Instrument [Line Items]
Long-term debt issued	3.3		0.4
Interest rate	1.00%		1.00%
IPL [Member]
Debt Instrument [Line Items]
Long-term debt issued	1,775.0		1,563.4
Current maturities	(150.0)		(38.4)
Unamortized debt (discount) and premium, net	(6.3)		(5.0)
Long-term debt, net	1,618.7		1,520.0
Repayments of commercial paper	60
Payments to retire long-term debt	(38.4)		0		0
IPL [Member] | Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt issued	1,775.0		1,525.0
IPL [Member] | Senior Debentures [Member] | 3.3% senior debenture, due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued	150.0		150.0
Interest rate	3.30%		3.30%
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	[1]	0	[1]
Interest rate	3.25%
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] | Pollution Control Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term debt issued	0		38.4
IPL [Member] | Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond [Member]
Debt Instrument [Line Items]
Long-term debt issued	0	[4]	38.4	[4]
Interest rate			5.00%
WPL [Member]
Debt Instrument [Line Items]
Long-term debt issued	1,580.6		1,339.1
Current maturities	(30.6)		(8.5)
Unamortized debt (discount) and premium, net	(6.7)		(7.0)
Long-term debt, net	1,543.3		1,323.6
WPL [Member] | Debentures [Member]
Debt Instrument [Line Items]
Long-term debt issued	1,550.0		1,300.0
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	[2]	0	[2]
Interest rate	4.10%
WPL [Member] | Pollution Control Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term debt issued	30.6		39.1
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond, due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued	16.0	[3]	24.5	[3]
Interest rate	5.00%		5.00%
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bond, due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt issued	14.6		14.6
Interest rate	5.375%		5.375%
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond [Member]
Debt Instrument [Line Items]
Payments to retire long-term debt	(8.5)
Franklin County Holdings [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through 2016, 1% at December 31, 2014 [Member]
Debt Instrument [Line Items]
Long-term debt issued	60.0	[6]	0	[6]
Interest rate	1.00%
Franklin County Holdings [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through 2014, 1% at December 31, 2013 [Member]
Debt Instrument [Line Items]
Long-term debt issued	$ 0	[6]	$ 60.0	[6]
Interest rate			1.00%

[1] In 2014, IPL issued $250.0 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce cash proceeds received from its sales of accounts receivable program, reduce commercial paper classified as long-term debt by $60 million and for general corporate purposes.

[2] In 2014, WPL issued $250.0 million of 4.1% debentures due 2044. The proceeds from the issuance were used by WPL to reduce commercial paper and for general corporate purposes.

[3] In 2014, WPL retired $8.5 million of its 5% pollution control revenue bonds.

[4] In 2014, IPL retired its $38.4 million, 5% pollution control revenue bonds.

[5] In 2014, Alliant Energy entered into a $250.0 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement to retire its $250.0 million, 4% senior notes due 2014.

[6] In 2014, Franklin County Holdings LLC, Resources’ wholly-owned subsidiary, entered into a $60.0 million variable-rate term loan credit agreement and used the proceeds to retire its borrowings under a term loan credit agreement that matured in December 2014.