Debt (Tables)

3 Months Ended

Mar. 31, 2015

Debt Disclosure [Line Items]

Other Short-Term Borrowings

Information regarding commercial paper classified as short-term debt and back-stopped by the credit facilities was as follows (dollars in millions):

	Alliant Energy	Parent
March 31, 2015	(Consolidated)	Company	IPL	WPL
Commercial paper:
Amount outstanding	$41.5	$41.5	$—	$—
Weighted average remaining maturity	1 day	1 day	N/A	N/A
Weighted average interest rates	0.5%	0.5%	N/A	N/A
Available credit facility capacity	$958.5	$258.5	$300.0	$400.0

	Alliant Energy						IPL						WPL
Three Months Ended March 31	2015			2014			2015			2014			2015		2014
Maximum amount outstanding (based on daily outstanding balances)		$151.3			$316.2			$1.4			$10.0			$—		$204.7
Average amount outstanding (based on daily outstanding balances)		$127.9			$275.6			$—			$0.3			$—		$173.0
Weighted average interest rates	0.4		%	0.1		%	0.4		%	0.2		%	N/A		0.1		%

IPL [Member]

Debt Disclosure [Line Items]

Other Short-Term Borrowings

Information regarding commercial paper classified as short-term debt and back-stopped by the credit facilities was as follows (dollars in millions):

	Alliant Energy	Parent
March 31, 2015	(Consolidated)	Company	IPL	WPL
Commercial paper:
Amount outstanding	$41.5	$41.5	$—	$—
Weighted average remaining maturity	1 day	1 day	N/A	N/A
Weighted average interest rates	0.5%	0.5%	N/A	N/A
Available credit facility capacity	$958.5	$258.5	$300.0	$400.0

	Alliant Energy						IPL						WPL
Three Months Ended March 31	2015			2014			2015			2014			2015		2014
Maximum amount outstanding (based on daily outstanding balances)		$151.3			$316.2			$1.4			$10.0			$—		$204.7
Average amount outstanding (based on daily outstanding balances)		$127.9			$275.6			$—			$0.3			$—		$173.0
Weighted average interest rates	0.4		%	0.1		%	0.4		%	0.2		%	N/A		0.1		%

WPL [Member]

Debt Disclosure [Line Items]

Other Short-Term Borrowings

Information regarding commercial paper classified as short-term debt and back-stopped by the credit facilities was as follows (dollars in millions):

	Alliant Energy	Parent
March 31, 2015	(Consolidated)	Company	IPL	WPL
Commercial paper:
Amount outstanding	$41.5	$41.5	$—	$—
Weighted average remaining maturity	1 day	1 day	N/A	N/A
Weighted average interest rates	0.5%	0.5%	N/A	N/A
Available credit facility capacity	$958.5	$258.5	$300.0	$400.0

	Alliant Energy						IPL						WPL
Three Months Ended March 31	2015			2014			2015			2014			2015		2014
Maximum amount outstanding (based on daily outstanding balances)		$151.3			$316.2			$1.4			$10.0			$—		$204.7
Average amount outstanding (based on daily outstanding balances)		$127.9			$275.6			$—			$0.3			$—		$173.0
Weighted average interest rates	0.4		%	0.1		%	0.4		%	0.2		%	N/A		0.1		%