Regulatory Matters (Tables)

12 Months Ended

Dec. 31, 2018

Regulatory Assets [Line Items]

Schedule of Regulatory Assets

At December 31, regulatory assets were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	2018		2017		2018		2017		2018		2017
Tax-related		$820.6		$778.2		$783.1		$750.5		$37.5		$27.7
Pension and OPEB costs	542.3		548.0		274.0		274.4		268.3		273.6
EGUs retired early	111.6		63.8		55.4		31.6		56.2		32.2
AROs	110.8		109.3		76.3		72.5		34.5		36.8
Derivatives	28.0		45.3		15.1		21.8		12.9		23.5
Emission allowances	23.6		25.5		23.6		25.5		—		—
Other	100.4		96.6		51.5		55.3		48.9		41.3
		$1,737.3		$1,666.7		$1,279.0		$1,231.6		$458.3		$435.1

Electric Generating Units Retired Early

Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows:

Entity	EGU	Retirement Date	Regulatory Asset Balance as of Dec. 31, 2018		Recovery	Regulatory Approval
IPL	Sutherland Units 1 and 3	June 2017		$27.6	Return of remaining net book value over 10 years	IUB and FERC
IPL	M.L. Kapp Unit 2	June 2018	27.8		Return of and return on remaining net book value over 10 years	Pending with FERC; to be addressed with IUB
WPL	Nelson Dewey Units 1 and 2 and Edgewater Unit 3	December 2015	27.4		Return of and return on remaining net book value over 10 years	PSCW and FERC
WPL	Edgewater Unit 4	September 2018	28.8		Return of and return on remaining net book value over 10 years	PSCW and pending with FERC

Schedule of Regulatory Liabilities

At December 31, regulatory liabilities were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	2018		2017		2018		2017		2018		2017
Tax-related		$890.6		$899.4		$390.1		$399.5		$500.5		$499.9
Cost of removal obligations	401.2		410.0		273.3		274.5		127.9		135.5
Electric transmission cost recovery	104.0		90.4		47.7		26.4		56.3		64.0
WPL earnings sharing mechanism	25.4		8.0		—		—		25.4		8.0
Commodity cost recovery	16.8		21.0		11.9		14.6		4.9		6.4
IPL’s tax benefit riders	6.4		25.0		6.4		25.0		—		—
Other	48.8		43.4		25.5		15.4		23.3		28.0
		$1,493.2		$1,497.2		$754.9		$755.4		$738.3		$741.8

Tax Benefit Riders

In 2018, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions):

Electric tax benefit rider credits		$17
Gas tax benefit rider credits	2
		$19

IPL [Member]

Regulatory Assets [Line Items]

Schedule of Regulatory Assets

At December 31, regulatory assets were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	2018		2017		2018		2017		2018		2017
Tax-related		$820.6		$778.2		$783.1		$750.5		$37.5		$27.7
Pension and OPEB costs	542.3		548.0		274.0		274.4		268.3		273.6
EGUs retired early	111.6		63.8		55.4		31.6		56.2		32.2
AROs	110.8		109.3		76.3		72.5		34.5		36.8
Derivatives	28.0		45.3		15.1		21.8		12.9		23.5
Emission allowances	23.6		25.5		23.6		25.5		—		—
Other	100.4		96.6		51.5		55.3		48.9		41.3
		$1,737.3		$1,666.7		$1,279.0		$1,231.6		$458.3		$435.1

Electric Generating Units Retired Early

Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows:

Entity	EGU	Retirement Date	Regulatory Asset Balance as of Dec. 31, 2018		Recovery	Regulatory Approval
IPL	Sutherland Units 1 and 3	June 2017		$27.6	Return of remaining net book value over 10 years	IUB and FERC
IPL	M.L. Kapp Unit 2	June 2018	27.8		Return of and return on remaining net book value over 10 years	Pending with FERC; to be addressed with IUB
WPL	Nelson Dewey Units 1 and 2 and Edgewater Unit 3	December 2015	27.4		Return of and return on remaining net book value over 10 years	PSCW and FERC
WPL	Edgewater Unit 4	September 2018	28.8		Return of and return on remaining net book value over 10 years	PSCW and pending with FERC

Schedule of Regulatory Liabilities

At December 31, regulatory liabilities were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	2018		2017		2018		2017		2018		2017
Tax-related		$890.6		$899.4		$390.1		$399.5		$500.5		$499.9
Cost of removal obligations	401.2		410.0		273.3		274.5		127.9		135.5
Electric transmission cost recovery	104.0		90.4		47.7		26.4		56.3		64.0
WPL earnings sharing mechanism	25.4		8.0		—		—		25.4		8.0
Commodity cost recovery	16.8		21.0		11.9		14.6		4.9		6.4
IPL’s tax benefit riders	6.4		25.0		6.4		25.0		—		—
Other	48.8		43.4		25.5		15.4		23.3		28.0
		$1,493.2		$1,497.2		$754.9		$755.4		$738.3		$741.8

Tax Benefit Riders

In 2018, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions):

Electric tax benefit rider credits		$17
Gas tax benefit rider credits	2
		$19

WPL [Member]

Regulatory Assets [Line Items]

Schedule of Regulatory Assets

At December 31, regulatory assets were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	2018		2017		2018		2017		2018		2017
Tax-related		$820.6		$778.2		$783.1		$750.5		$37.5		$27.7
Pension and OPEB costs	542.3		548.0		274.0		274.4		268.3		273.6
EGUs retired early	111.6		63.8		55.4		31.6		56.2		32.2
AROs	110.8		109.3		76.3		72.5		34.5		36.8
Derivatives	28.0		45.3		15.1		21.8		12.9		23.5
Emission allowances	23.6		25.5		23.6		25.5		—		—
Other	100.4		96.6		51.5		55.3		48.9		41.3
		$1,737.3		$1,666.7		$1,279.0		$1,231.6		$458.3		$435.1

Electric Generating Units Retired Early

Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows:

Entity	EGU	Retirement Date	Regulatory Asset Balance as of Dec. 31, 2018		Recovery	Regulatory Approval
IPL	Sutherland Units 1 and 3	June 2017		$27.6	Return of remaining net book value over 10 years	IUB and FERC
IPL	M.L. Kapp Unit 2	June 2018	27.8		Return of and return on remaining net book value over 10 years	Pending with FERC; to be addressed with IUB
WPL	Nelson Dewey Units 1 and 2 and Edgewater Unit 3	December 2015	27.4		Return of and return on remaining net book value over 10 years	PSCW and FERC
WPL	Edgewater Unit 4	September 2018	28.8		Return of and return on remaining net book value over 10 years	PSCW and pending with FERC

Schedule of Regulatory Liabilities

At December 31, regulatory liabilities were comprised of the following items (in millions):

	Alliant Energy				IPL				WPL
	2018		2017		2018		2017		2018		2017
Tax-related		$890.6		$899.4		$390.1		$399.5		$500.5		$499.9
Cost of removal obligations	401.2		410.0		273.3		274.5		127.9		135.5
Electric transmission cost recovery	104.0		90.4		47.7		26.4		56.3		64.0
WPL earnings sharing mechanism	25.4		8.0		—		—		25.4		8.0
Commodity cost recovery	16.8		21.0		11.9		14.6		4.9		6.4
IPL’s tax benefit riders	6.4		25.0		6.4		25.0		—		—
Other	48.8		43.4		25.5		15.4		23.3		28.0
		$1,493.2		$1,497.2		$754.9		$755.4		$738.3		$741.8