Condensed Consolidated Statements Of Cash Flows (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Cash flows from operating activities:
Net income	$ 263.4	$ 260.9
Adjustments to reconcile net income (loss) to net cash flows from operating activities:
Depreciation and amortization	247.9	242.3
Other amortizations	41.3	42.1
Deferred tax expense (benefit) and investment tax credits	85.6	30.7
Equity income from unconsolidated investments, net	(30.4)	(29.6)
Distributions from equity method investments	25.7	24.4
Other	(8.4)	13.5
Other changes in assets and liabilities:
Accounts receivable	50.9	(49.7)
Sales of accounts receivable	(85.0)	50.0
Production fuel	(8.6)	27.7
Regulatory assets	(73.5)	(191.1)
Accounts payable	47.2	25.9
Regulatory liabilities	5.0	158.5
Pension and other benefit obligations	(1.9)	(67.9)
Other	(49.4)	(21.8)
Net cash flows from operating activities	600.3	612.7
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(412.7)	(480.2)
Alliant Energy Corporate Services, Inc. and non-regulated businesses	(106.3)	(46.1)
Other	1.3	19.9
Net cash flows used for investing activities	(517.7)	(506.4)
Cash flows from (used for) financing activities:
Common stock dividends	(149.6)	(141.1)
Preferred dividends paid by subsidiaries	(11.9)	(12.9)
Payments to redeem cumulative preferred stock of IPL	0	(40.0)
Proceeds from issuance of long-term debt	75.0	0.4
Net change in commercial paper	17.6	(25.3)
Other	16.0	(1.0)
Net cash flows from (used for) financing activities	(52.9)	(219.9)
Net increase (decrease) in cash and cash equivalents	29.7	(113.6)
Cash and cash equivalents at beginning of period	11.4	159.3
Cash and cash equivalents at end of period	41.1	45.7
Supplemental cash flows information:
Interest, net of capitalized interest	115.6	119.0
Income taxes, net of refunds	(0.8)	(3.0)
Significant noncash investing and financing activities:
Accrued capital expenditures	123.8	27.7
IPL [Member]
Cash flows from operating activities:
Net income	124.6	129.5
Adjustments to reconcile net income (loss) to net cash flows from operating activities:
Depreciation and amortization	141.1	134.1
Deferred tax expense (benefit) and investment tax credits	(9.2)	(31.1)
Other	5.4	17.6
Other changes in assets and liabilities:
Accounts receivable	(2.6)	14.3
Sales of accounts receivable	(85.0)	50.0
Production fuel	(8.4)	19.3
Regulatory assets	(57.7)	(202.0)
Accounts payable	27.4	(2.3)
Accrued taxes	(5.8)	(28.2)
Regulatory liabilities	(27.5)	145.1
Pension and other benefit obligations	(2.1)	(37.5)
Other	(16.7)	10.7
Net cash flows from operating activities	172.4	314.1
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(194.6)	(232.3)
Proceeds from sale of wind project assets to affiliate	0	115.3
Other	(16.5)	(15.2)
Net cash flows used for investing activities	(211.1)	(132.2)
Cash flows from (used for) financing activities:
Common stock dividends	(91.6)	(43.7)
Preferred dividends paid by subsidiaries	(9.4)	(10.4)
Capital contributions from parent	80.0	0
Repayment of capital to parent	0	(100.7)
Payments to redeem cumulative preferred stock of IPL	0	(40.0)
Net change in commercial paper	42.9	0
Changes in cash overdrafts	20.0	7.3
Other	0	0.1
Net cash flows from (used for) financing activities	41.9	(187.4)
Net increase (decrease) in cash and cash equivalents	3.2	(5.5)
Cash and cash equivalents at beginning of period	2.1	5.7
Cash and cash equivalents at end of period	5.3	0.2
Supplemental cash flows information:
Interest, net of capitalized interest	58.5	58.3
Income taxes, net of refunds	(11.3)	15.0
Significant noncash investing and financing activities:
Accrued capital expenditures	69.2	11.3
WPL [Member]
Cash flows from operating activities:
Net income	124.7	120.8
Adjustments to reconcile net income (loss) to net cash flows from operating activities:
Depreciation and amortization	104.8	104.5
Other amortizations	32.7	31.8
Deferred tax expense (benefit) and investment tax credits	89.3	77.3
Equity income from unconsolidated investments, net	(31.2)	(28.9)
Distributions from equity method investments	25.7	24.4
Other	(8.9)	6.9
Other changes in assets and liabilities:
Accounts receivable	9.9	27.5
Income tax refunds receivable	(22.3)	25.1
Regulatory assets	(15.8)	10.9
Regulatory liabilities	32.5	13.4
Pension and other benefit obligations	(0.2)	(15.5)
Other	(3.9)	(21.8)
Net cash flows from operating activities	339.6	381.3
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(218.1)	(247.9)
Other	5.6	5.5
Net cash flows used for investing activities	(212.5)	(242.4)
Cash flows from (used for) financing activities:
Common stock dividends	(84.1)	(83.3)
Preferred dividends paid by subsidiaries	(2.5)	(2.5)
Capital contributions from parent	0	25.0
Net change in commercial paper	(25.7)	(47.4)
Other	(1.3)	(7.3)
Net cash flows from (used for) financing activities	(113.6)	(115.5)
Net increase (decrease) in cash and cash equivalents	13.5	23.4
Cash and cash equivalents at beginning of period	2.7	0.1
Cash and cash equivalents at end of period	16.2	23.5
Supplemental cash flows information:
Interest, net of capitalized interest	63.1	63.4
Income taxes, net of refunds	7.9	(37.1)
Significant noncash investing and financing activities:
Accrued capital expenditures	45.0	15.1
Deferred income tax assets [Member]
Other changes in assets and liabilities:
Deferred income taxes	(67.2)	(0.6)
Deferred income tax assets [Member] | IPL [Member]
Other changes in assets and liabilities:
Deferred income taxes	(22.4)	3.1
Deferred income tax assets [Member] | WPL [Member]
Other changes in assets and liabilities:
Deferred income taxes	(44.3)	(1.3)
Deferred income tax liabilities [Member]
Other changes in assets and liabilities:
Deferred income taxes	157.7	97.4
Deferred income tax liabilities [Member] | IPL [Member]
Other changes in assets and liabilities:
Deferred income taxes	111.3	91.5
Deferred income tax liabilities [Member] | WPL [Member]
Other changes in assets and liabilities:
Deferred income taxes	$ 46.6	$ 6.2