Condensed Consolidated Statements Of Cash Flows (USD $) In Millions	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 IPL [Member]	Jun. 30, 2010 IPL [Member]	Jun. 30, 2011 WPL [Member]	Jun. 30, 2010 WPL [Member]
Cash flows from operating activities:
Net income	$ 135.0	$ 100.6	$ 28.1	$ 30.9	$ 69.4	$ 67.2
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	161.0	140.4
Depreciation and amortization	161.0	139.5	88.9	86.6	69.5	49.8
Other amortizations	28.3	23.1			21.2	19.7
Deferred tax expense (benefit) and investment tax credits	(25.5)	48.1	(41.4)	25.4	34.3	24.1
Equity income from unconsolidated investments, net	(19.5)	(19.4)			(19.1)	(19.1)
Distributions from equity method investments	15.9	16.2			15.9	16.2
Other	16.7	(0.1)	15.4	6.8	8.1	(3.7)
Other changes in assets and liabilities:
Accounts receivable	27.8	64.6	35.1	39.8	19.9	19.8
Sales of accounts receivable	55.0	120.0	55.0	120.0
Income tax refunds receivable					33.7	(6.2)
Regulatory assets	(110.3)	(56.6)	(133.3)	(37.7)	23.0	(18.9)
Regulatory liabilities	165.9	(20.2)	158.0	(8.1)
Deferred income taxes	70.8	9.5	65.8	7.6
Pension and other benefit obligations	(62.0)	36.7	(35.3)	14.7	(15.5)	11.6
Other	7.7	16.3	8.6	(0.5)	10.1	6.2
Net cash flows from operating activities	466.8	479.2	244.9	285.5	270.5	166.7
Cash flows used for investing activities:
Utility business	(338.1)	(384.8)	(163.5)	(183.0)	(174.6)	(201.8)
Alliant Energy Corporate Services, Inc. and non-regulated businesses	(20.6)	(11.2)
Other	17.6	0.4	(11.7)	(18.0)	3.3	2.5
Net cash flows used for investing activities	(341.1)	(395.6)	(59.9)	(201.0)	(171.3)	(199.3)
Cash flows used for financing activities:
Common stock dividends	(94.2)	(87.2)	(43.7)		(55.5)	(56.0)
Preferred dividends paid by subsidiaries	(8.9)	(9.4)	(7.2)	(7.7)	(1.7)	(1.7)
Capital contributions from parent				50.0	25.0	50.0
Repayment of capital to parent			(71.0)	(58.7)
Payments to redeem cumulative preferred stock	(40.0)		(40.0)
Proceeds from issuance of long-term debt	0.4	300.0		150.0		150.0
Payments to retire long-term debt	(0.6)	(100.8)				(100.0)
Net change in short-term borrowings	(47.4)	(190.0)		(190.0)	(47.4)
Other	(17.2)	(0.4)	(7.8)		(6.5)	(0.5)
Net cash flows used for financing activities	(207.9)	(87.8)	(169.7)	(56.4)	(86.1)	41.8
Net increase in cash and cash equivalents	(82.2)	(4.2)	15.3	28.1	13.1	9.2
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	77.1	171.1	21.0	28.5	13.2	27.7
Supplemental cash flows information:
Interest, net of capitalized interest	80.2	82.5	39.0	39.9	40.0	41.1
Income taxes, net of refunds	(3.0)	(6.7)	15.5	(22.4)	(35.8)	21.7
Significant noncash investing and financing activities:
Accrued capital expenditures	$ 43.3	$ 47.0	$ 19.1	$ 24.6	$ 16.9	$ 21.7