Condensed Consolidated Statements Of Cash Flows (USD $) In Millions	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 IPL [Member]	Sep. 30, 2010 IPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2010 WPL [Member]
Cash flows from operating activities:
Net income	$ 260.9	$ 254.3	$ 129.5	$ 131.5	$ 120.8	$ 117.2
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	242.3	215.9
Depreciation and amortization	242.3	214.9	134.1	131.4	104.5	79.0
Other amortizations	42.1	37.1			31.8	29.6
Deferred tax expense (benefit) and investment tax credits	30.7	177.6	(31.1)	98.8	77.3	72.4
Equity income from unconsolidated investments, net	(29.6)	(28.7)			(28.9)	(28.4)
Distributions from equity method investments	24.4	24.5			24.4	24.5
Other	13.5	2.1	17.6	8.6	6.9	(6.2)
Other changes in assets and liabilities:
Accounts receivable	(49.7)	23.0	14.3	13.8	27.5	20.8
Sales of accounts receivable	50.0	100.0	50.0	100.0
Income tax refunds receivable	(8.1)	33.5	0.1	17.8	25.1	26.7
Regulatory assets	(191.1)	(108.4)	(202.0)	(85.6)	10.9	(22.8)
Accounts payable	25.9	(12.1)	(2.3)	(22.5)	(10.0)	10.4
Regulatory liabilities	158.5	18.6	145.1	27.3	13.4	(8.7)
Derivative liabilities					(11.6)	0.8
Accrued taxes	(8.1)	(40.9)	(28.2)	(35.1)
Deferred income taxes	97.4	20.9	91.5	10.4
Non-current taxes payable	(3.8)	(66.8)	0.2	(28.3)	(0.3)	(38.8)
Pension and other benefit obligations	(67.9)	14.9	(37.5)	13.1	(15.5)	8.1
Other	25.3	30.1	32.8	(2.2)	5.0	9.2
Net cash flows from operating activities	612.7	695.6	314.1	379.0	381.3	293.8
Cash flows used for investing activities:
Utility business	(480.2)	(587.6)	(232.3)	(272.5)	(247.9)	(315.1)
Proceeds from sale of wind project assets to affiliate			115.3
Alliant Energy Corporate Services, Inc. and non-regulated businesses	(46.1)	(22.4)
Advances for customer energy efficiency projects	(4.6)	(11.3)			(4.6)	(11.3)
Collections of advances for customer energy efficiency projects	24.5	25.9			20.7	22.8
Other		(11.1)	(15.2)	(23.7)	(10.6)	(9.7)
Net cash flows used for investing activities	(506.4)	(606.5)	(132.2)	(296.2)	(242.4)	(313.3)
Cash flows used for financing activities:
Common stock dividends	(141.1)	(130.9)	(43.7)		(83.3)	(82.2)
Preferred dividends paid by subsidiaries	(12.9)	(14.0)	(10.4)	(11.5)	(2.5)	(2.5)
Capital contributions from parent				50.0	25.0	75.0
Repayment of capital to parent			(100.7)	(88.5)
Payments to redeem cumulative preferred stock	(40.0)		(40.0)
Proceeds from issuance of long-term debt	0.4	500.0		350.0		150.0
Payments to retire long-term debt	(0.6)	(307.1)		(206.3)		(100.0)
Net change in short-term borrowings	(25.3)	(190.0)		(190.0)	(47.4)
Changes in cash overdrafts			7.3	18.7
Other	(0.4)	14.6	0.1	(3.0)	(7.3)	(3.8)
Net cash flows used for financing activities	(219.9)	(127.4)	(187.4)	(80.6)	(115.5)	36.5
Net decrease in cash and cash equivalents	(113.6)	(38.3)	(5.5)	2.2	23.4	17.0
Cash and cash equivalents at beginning of period	159.3	175.3	5.7	0.4	0.1	18.5
Cash and cash equivalents at end of period	45.7	137.0	0.2	2.6	23.5	35.5
Supplemental cash flows information:
Interest, net of capitalized interest	119.0	125.4	58.3	62.2	63.4	64.5
Income taxes, net of refunds	(3.0)	(6.4)	15.0	(19.8)	(37.1)	13.9
Significant noncash investing and financing activities:
Accrued capital expenditures	$ 27.7	$ 38.0	$ 11.3	$ 15.5	$ 15.1	$ 21.5