Consolidated Statements of Common Equity (USD $) In Millions, unless otherwise specified	Total	IPL [Member]	WPL [Member]	Common Stock [Member]	Common Stock [Member] IPL [Member]	Common Stock [Member] WPL [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] IPL [Member]	Additional Paid-in Capital [Member] WPL [Member]	Retained Earnings [Member]	Retained Earnings [Member] IPL [Member]	Retained Earnings [Member] WPL [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Deferred Compensation, Share-based Payments [Member]
Beginning balance at Dec. 31, 2009	$ 2,772.6	$ 1,330.0	$ 1,254.7	$ 1.1	$ 33.4	$ 66.2	$ 1,499.1	$ 1,042.2	$ 768.9	$ 1,281.7	$ 254.4	$ 419.6	$ (1.4)	$ (7.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Alliant Energy common shareowners	287.6	128.0	149.0							287.6	128.0	149.0
Common stock dividends	174.6		109.5							174.6		109.5
Capital contributions from parent		50.0	75.0					50.0	75.0
Repayment of capital to parent		(118.2)						(118.2)
Common stock issued, repurchased and other, net	8.0		0.1				7.7		0.1					0.3
Ending balance at Dec. 31, 2010	2,893.6	1,389.8	1,369.3	1.1	33.4	66.2	1,506.8	974.0	844.0	1,394.7	382.4	459.1		(7.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Alliant Energy common shareowners	73.5	21.7	43.6
Ending balance at Mar. 31, 2011
Beginning balance at Dec. 31, 2010	2,893.6	1,389.8	1,369.3	1.1	33.4	66.2	1,506.8	974.0	844.0	1,394.7	382.4	459.1	(1.4)	(7.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Alliant Energy common shareowners	303.6	124.3	160.2							303.6	124.3	160.2
Common stock dividends	188.1	73.4	112.1							188.1	73.4	112.1
Capital contributions from parent		54.0	25.0					54.0	25.0
Repayment of capital to parent		(100.7)						(100.7)
Common stock issued, repurchased and other, net	3.3	0.4					4.0	0.4						(0.7)
Other comprehensive income, net of tax	0.6												0.6
Ending balance at Dec. 31, 2011	3,013.0	1,394.4	1,442.4	1.1	33.4	66.2	1,510.8	927.7	869.0	1,510.2	433.3	507.2	(0.8)	(8.3)
Beginning balance at Sep. 30, 2011
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Alliant Energy common shareowners	57.0	6.6	41.9
Ending balance at Dec. 31, 2011	3,013.0	1,394.4	1,442.4	1.1	33.4	66.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Alliant Energy common shareowners	34.9	(4.7)	31.1
Ending balance at Mar. 31, 2012
Beginning balance at Dec. 31, 2011	3,013.0	1,394.4	1,442.4	1.1	33.4	66.2	1,510.8	927.7	869.0	1,510.2	433.3	507.2		(8.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Alliant Energy common shareowners	319.8	137.6	162.4							319.8	137.6	162.4
Common stock dividends	199.3	122.9	112.0							199.3	122.9	112.0
Capital contributions from parent		110.0	90.0					110.0	90.0
Common stock issued, repurchased and other, net	1.4	0.1	0.2				0.4	0.1	0.2					1.0
Ending balance at Dec. 31, 2012	3,134.9	1,519.2	1,583.0	1.1	33.4	66.2	1,511.2	1,037.8	959.2	1,630.7	448.0	557.6	(0.8)	(7.3)
Beginning balance at Sep. 30, 2012
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Alliant Energy common shareowners	68.3	22.4	40.2
Ending balance at Dec. 31, 2012	$ 3,134.9	$ 1,519.2	$ 1,583.0	$ 1.1	$ 33.4	$ 66.2							$ (0.8)