Condensed Consolidated Statements Of Cash Flows (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash flows from operating activities:
Net income (loss)	$ 80.1	$ 38.9
Adjustments to reconcile net income (loss) to net cash flows from operating activities:
Depreciation and amortization	92.6	83.6
Other amortizations	9.7	14.0
Deferred tax expense (benefit) and investment tax credits	22.5	31.9
Equity income from unconsolidated investments, net	(10.7)	(9.4)
Distributions from equity method investments	8.9	8.6
Other	(3.6)	(1.8)
Other changes in assets and liabilities:
Accounts receivable	(12.9)	63.9
Sales of accounts receivable	(30.0)	5.0
Gas stored underground	28.7	30.3
Regulatory assets	19.9	(18.9)
Regulatory liabilities	20.4	(26.5)
Derivative liabilities	(23.3)	5.2
Other	(3.7)	(9.8)
Net cash flows from operating activities	198.6	215.0
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(153.2)	(122.1)
Alliant Energy Corporate Services, Inc. and non-regulated businesses	(14.1)	(13.5)
Proceeds from Franklin County wind project cash grant	62.4	0
Other	(14.8)	0.5
Net cash flows used for investing activities	(119.7)	(135.1)
Cash flows from (used for) financing activities:
Common stock dividends	(52.2)	(49.9)
Preferred dividends paid by subsidiaries	(3.8)	(4.0)
Payments to redeem cumulative preferred stock	(211.0)	0
Proceeds from issuance of cumulative preferred stock	200.0	0
Net change in commercial paper	10.9	(20.8)
Other	9.4	14.3
Net cash flows from (used for) financing activities	(46.7)	(60.4)
Net increase (decrease) in cash and cash equivalents	32.2	19.5
Cash and cash equivalents at beginning of period	21.2	11.4
Cash and cash equivalents at end of period	53.4	30.9
Supplemental cash flows information:
Interest, net of capitalized interest	43.5	38.8
Income taxes, net of refunds	2.3	(0.1)
Significant non-cash investing and financing activities:
Accrued capital expenditures	99.5	42.0
IPL [Member]
Cash flows from operating activities:
Net income (loss)	31.5	(1.5)
Adjustments to reconcile net income (loss) to net cash flows from operating activities:
Depreciation and amortization	47.6	46.7
Deferred tax expense (benefit) and investment tax credits	(10.7)	7.0
Other	(0.7)	1.3
Other changes in assets and liabilities:
Accounts receivable	(34.7)	18.3
Sales of accounts receivable	(30.0)	5.0
Income tax refunds receivable	(8.7)	13.8
Gas stored underground	15.0	18.7
Regulatory liabilities	14.7	(33.6)
Other	19.3	(8.2)
Net cash flows from operating activities	65.2	78.1
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(77.5)	(56.6)
Other	(5.7)	(4.8)
Net cash flows used for investing activities	(83.2)	(61.4)
Cash flows from (used for) financing activities:
Common stock dividends	(31.6)	(29.7)
Preferred dividends paid by subsidiaries	(3.2)	(3.2)
Capital contributions from parent	30.0	0
Payments to redeem cumulative preferred stock	(150.0)	0
Proceeds from issuance of cumulative preferred stock	200.0	0
Net change in commercial paper	(41.3)	17.9
Changes in cash overdrafts	20.1	19.3
Other	(5.8)	0
Net cash flows from (used for) financing activities	18.2	4.3
Net increase (decrease) in cash and cash equivalents	0.2	21.0
Cash and cash equivalents at beginning of period	4.5	2.1
Cash and cash equivalents at end of period	4.7	23.1
Supplemental cash flows information:
Interest, net of capitalized interest	21.1	19.4
Income taxes, net of refunds	4.8	(14.4)
Significant non-cash investing and financing activities:
Accrued capital expenditures	60.7	23.5
WPL [Member]
Cash flows from operating activities:
Net income (loss)	43.6	31.9
Adjustments to reconcile net income (loss) to net cash flows from operating activities:
Depreciation and amortization	43.1	35.8
Other amortizations	7.1	11.0
Deferred tax expense (benefit) and investment tax credits	26.7	23.7
Equity income from unconsolidated investments, net	(10.8)	(10.1)
Distributions from equity method investments	8.9	8.6
Other	(1.3)	(0.4)
Other changes in assets and liabilities:
Income tax refunds receivable	(15.1)	(6.4)
Gas stored underground	13.7	11.6
Regulatory assets	13.3	(9.9)
Accrued taxes	(24.3)	(3.2)
Derivative liabilities	(13.3)	0.7
Other	13.8	1.0
Net cash flows from operating activities	105.4	94.3
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(75.7)	(65.5)
Other	(0.4)	1.9
Net cash flows used for investing activities	(76.1)	(63.6)
Cash flows from (used for) financing activities:
Common stock dividends	(29.1)	(28.1)
Preferred dividends paid by subsidiaries	(0.6)	(0.8)
Payments to redeem cumulative preferred stock	(61.0)	0
Net change in commercial paper	73.4	(2.4)
Other	(4.3)	0
Net cash flows from (used for) financing activities	(21.6)	(31.3)
Net increase (decrease) in cash and cash equivalents	7.7	(0.6)
Cash and cash equivalents at beginning of period	0.7	2.7
Cash and cash equivalents at end of period	8.4	2.1
Supplemental cash flows information:
Interest, net of capitalized interest	23.4	23.5
Income taxes, net of refunds	25.9	12.2
Significant non-cash investing and financing activities:
Accrued capital expenditures	35.0	16.3
Deferred income tax assets [Member]
Other changes in assets and liabilities:
Deferred income tax liabilities	77.7
Deferred income tax assets [Member] | IPL [Member]
Other changes in assets and liabilities:
Deferred income tax liabilities	20.6
Deferred income tax assets [Member] | WPL [Member]
Other changes in assets and liabilities:
Deferred income tax liabilities	63.0
Deferred income tax liabilities [Member] | IPL [Member]
Other changes in assets and liabilities:
Deferred income tax liabilities	$ 21.9	$ 10.6