Summary Of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2013

Regulatory Assets

Regulatory assets were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012
Tax-related		$781.9		$770.7		$757.2		$746.2		$24.7		$24.5
Pension and other postretirement benefits costs	541.4		549.2		275.6		279.3		265.8		269.9
Asset retirement obligations (AROs)	62.9		62.4		38.5		38.6		24.4		23.8
Environmental-related costs	32.2		34.9		27.7		30.3		4.5		4.6
Emission allowances	30.0		30.0		30.0		30.0		—		—
Derivatives	17.0		40.2		6.3		16.3		10.7		23.9
Other	117.2		125.0		72.6		77.2		44.6		47.8
		$1,582.6		$1,612.4		$1,207.9		$1,217.9		$374.7		$394.5

Regulatory Liabilities

Regulatory liabilities were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012
Cost of removal obligations		$411.3		$408.7		$270.6		$268.0		$140.7		$140.7
IPL’s tax benefit riders	335.1		355.8		335.1		355.8		—		—
Energy conservation cost recovery	61.3		55.1		16.4		10.0		44.9		45.1
IPL’s electric transmission assets sale	29.7		32.5		29.7		32.5		—		—
Commodity cost recovery	39.2		17.7		25.4		5.2		13.8		12.5
Other	59.1		46.3		38.0		29.9		21.1		16.4
		$935.7		$916.1		$715.2		$701.4		$220.5		$214.7

IPL [Member]

Regulatory Assets

Regulatory assets were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012
Tax-related		$781.9		$770.7		$757.2		$746.2		$24.7		$24.5
Pension and other postretirement benefits costs	541.4		549.2		275.6		279.3		265.8		269.9
Asset retirement obligations (AROs)	62.9		62.4		38.5		38.6		24.4		23.8
Environmental-related costs	32.2		34.9		27.7		30.3		4.5		4.6
Emission allowances	30.0		30.0		30.0		30.0		—		—
Derivatives	17.0		40.2		6.3		16.3		10.7		23.9
Other	117.2		125.0		72.6		77.2		44.6		47.8
		$1,582.6		$1,612.4		$1,207.9		$1,217.9		$374.7		$394.5

Regulatory Liabilities

Regulatory liabilities were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012
Cost of removal obligations		$411.3		$408.7		$270.6		$268.0		$140.7		$140.7
IPL’s tax benefit riders	335.1		355.8		335.1		355.8		—		—
Energy conservation cost recovery	61.3		55.1		16.4		10.0		44.9		45.1
IPL’s electric transmission assets sale	29.7		32.5		29.7		32.5		—		—
Commodity cost recovery	39.2		17.7		25.4		5.2		13.8		12.5
Other	59.1		46.3		38.0		29.9		21.1		16.4
		$935.7		$916.1		$715.2		$701.4		$220.5		$214.7

WPL [Member]

Regulatory Assets

Regulatory assets were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012
Tax-related		$781.9		$770.7		$757.2		$746.2		$24.7		$24.5
Pension and other postretirement benefits costs	541.4		549.2		275.6		279.3		265.8		269.9
Asset retirement obligations (AROs)	62.9		62.4		38.5		38.6		24.4		23.8
Environmental-related costs	32.2		34.9		27.7		30.3		4.5		4.6
Emission allowances	30.0		30.0		30.0		30.0		—		—
Derivatives	17.0		40.2		6.3		16.3		10.7		23.9
Other	117.2		125.0		72.6		77.2		44.6		47.8
		$1,582.6		$1,612.4		$1,207.9		$1,217.9		$374.7		$394.5

Regulatory Liabilities

Regulatory liabilities were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012		March 31, 2013		December 31, 2012
Cost of removal obligations		$411.3		$408.7		$270.6		$268.0		$140.7		$140.7
IPL’s tax benefit riders	335.1		355.8		335.1		355.8		—		—
Energy conservation cost recovery	61.3		55.1		16.4		10.0		44.9		45.1
IPL’s electric transmission assets sale	29.7		32.5		29.7		32.5		—		—
Commodity cost recovery	39.2		17.7		25.4		5.2		13.8		12.5
Other	59.1		46.3		38.0		29.9		21.1		16.4
		$935.7		$916.1		$715.2		$701.4		$220.5		$214.7