Condensed Consolidated Statements Of Cash Flows (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Cash flows from operating activities:
Net income	$ 108.7	$ 135.0
Adjustments to reconcile net income (loss) to net cash flows from operating activities:
Depreciation and amortization	164.3	161.0
Other amortizations	27.6	28.3
Deferred tax expense (benefit) and investment tax credits	56.3	(25.5)
Equity income from unconsolidated investments, net	(20.0)	(19.5)
Distributions from equity method investments	16.8	15.9
Other	(7.6)	16.7
Other changes in assets and liabilities:
Accounts receivable	54.0	27.8
Sales of accounts receivable	(27.0)	55.0
Regulatory assets	(19.6)	(110.3)
Regulatory liabilities	(43.5)	165.9
Pension and other benefit obligations	(2.8)	(62.0)
Other	5.3	8.8
Net cash flows from operating activities	339.2	466.8
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(247.3)	(338.1)
Alliant Energy Corporate Services, Inc. and non-regulated businesses	(75.1)	(20.6)
Other	0.6	17.6
Net cash flows used for investing activities	(321.8)	(341.1)
Cash flows from (used for) financing activities:
Common stock dividends	(99.7)	(94.2)
Preferred dividends paid by subsidiaries	(7.9)	(8.9)
Payments for redeem cumulative preferred stock of IPL	0	(40.0)
Net change in commercial paper	110.0	(47.4)
Other	(0.4)	(17.4)
Net cash flows from (used for) financing activities	2.0	(207.9)
Net increase (decrease) in cash and cash equivalents	19.4	(82.2)
Cash and cash equivalents at beginning of period	11.4	159.3
Cash and cash equivalents at end of period	30.8	77.1
Supplemental cash flows information:
Interest, net of capitalized interest	77.4	80.2
Income taxes, net of refunds	(0.9)	(3.0)
Significant noncash investing and financing activities:
Accrued capital expenditures	100.3	43.3
IPL [Member]
Cash flows from operating activities:
Net income	18.1	28.1
Adjustments to reconcile net income (loss) to net cash flows from operating activities:
Depreciation and amortization	93.8	88.9
Deferred tax expense (benefit) and investment tax credits	8.2	(41.4)
Other	1.8	15.4
Other changes in assets and liabilities:
Accounts receivable	(0.5)	35.1
Sales of accounts receivable	(27.0)	55.0
Regulatory assets	(12.1)	(133.3)
Derivative assets	(20.9)	(8.3)
Regulatory liabilities	(61.5)	158.0
Accrued taxes	23.4	(5.0)
Pension and other benefit obligations	(1.5)	(35.3)
Other	28.9	20.8
Net cash flows from operating activities	76.4	244.9
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(109.9)	(163.5)
Proceeds from sale of wind project assets to affiliate	0	115.3
Other	(10.4)	(11.7)
Net cash flows used for investing activities	(120.3)	(59.9)
Cash flows from (used for) financing activities:
Common stock dividends	(60.8)	(43.7)
Preferred dividends paid by subsidiaries	(6.2)	(7.2)
Capital contributions from parent	50.0	0
Repayment of capital to parent	0	(71.0)
Payments for redeem cumulative preferred stock of IPL	0	(40.0)
Net change in commercial paper	58.3	0
Other	3.3	(7.8)
Net cash flows from (used for) financing activities	44.6	(169.7)
Net increase (decrease) in cash and cash equivalents	0.7	15.3
Cash and cash equivalents at beginning of period	2.1	5.7
Cash and cash equivalents at end of period	2.8	21.0
Supplemental cash flows information:
Interest, net of capitalized interest	39.2	39.0
Income taxes, net of refunds	(15.4)	15.5
Significant noncash investing and financing activities:
Accrued capital expenditures	61.1	19.1
WPL [Member]
Cash flows from operating activities:
Net income	68.0	69.4
Adjustments to reconcile net income (loss) to net cash flows from operating activities:
Depreciation and amortization	69.0	69.5
Other amortizations	21.8	21.2
Deferred tax expense (benefit) and investment tax credits	43.3	34.3
Equity income from unconsolidated investments, net	(20.7)	(19.1)
Distributions from equity method investments	16.8	15.9
Other	(6.5)	8.1
Other changes in assets and liabilities:
Accounts receivable	8.4	19.9
Income tax refunds receivable	(10.9)	33.7
Regulatory assets	(7.5)	23.0
Other	0	(8.6)
Net cash flows from operating activities	181.9	270.5
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(137.4)	(174.6)
Other	4.0	3.3
Net cash flows used for investing activities	(133.4)	(171.3)
Cash flows from (used for) financing activities:
Common stock dividends	(56.0)	(55.5)
Preferred dividends paid by subsidiaries	(1.7)	(1.7)
Capital contributions from parent	0	25.0
Net change in commercial paper	9.9	(47.4)
Other	(0.9)	(6.5)
Net cash flows from (used for) financing activities	(48.7)	(86.1)
Net increase (decrease) in cash and cash equivalents	(0.2)	13.1
Cash and cash equivalents at beginning of period	2.7	0.1
Cash and cash equivalents at end of period	2.5	13.2
Supplemental cash flows information:
Interest, net of capitalized interest	40.0	40.0
Income taxes, net of refunds	15.8	(35.8)
Significant noncash investing and financing activities:
Accrued capital expenditures	20.8	16.9
Deferred income tax assets [Member]
Other changes in assets and liabilities:
Deferred income taxes	(60.1)	(1.1)
Deferred income tax assets [Member] | IPL [Member]
Other changes in assets and liabilities:
Deferred income taxes	(21.4)	1.1
Deferred income tax assets [Member] | WPL [Member]
Other changes in assets and liabilities:
Deferred income taxes	(37.9)	2.2
Deferred income tax liabilities [Member]
Other changes in assets and liabilities:
Deferred income taxes	86.8	70.8
Deferred income tax liabilities [Member] | IPL [Member]
Other changes in assets and liabilities:
Deferred income taxes	47.1	65.8
Deferred income tax liabilities [Member] | WPL [Member]
Other changes in assets and liabilities:
Deferred income taxes	$ 38.1	$ 1.0