Consolidated Statements Of Cash Flows (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities:
Net income	$ 335.7	$ 321.9	$ 306.3
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	332.9	323.8	292.3
Other amortizations	55.0	56.3	51.8
Deferred tax expense (benefit) and investment tax credits	143.3	10.2	216.6
Equity income from unconsolidated investments, net	(41.3)	(39.3)	(38.1)
Distributions from equity method investments	34.2	32.3	32.2
Equity component of allowance for funds used during construction	(14.1)	(7.6)	(11.2)
Non-cash valuation and regulatory-related charges	3.3	25.5	38.0
Other	(2.6)	(5.2)	(4.7)
Other changes in assets and liabilities:
Accounts receivable	61.3	(54.3)	16.3
Sales of accounts receivable	(10.0)	75.0	65.0
Income tax refunds receivable	20.9	0.3	130.4
Regulatory assets	(178.1)	(413.1)	(20.8)
Regulatory liabilities	16.4	168.3	8.4
Derivative liabilities	(37.6)	10.7	(52.0)
Non-current taxes payable	(2.9)	(5.1)	(66.6)
Pension and other benefit obligations	51.3	8.9	(20.1)
Other	(14.2)	45.6	12.2
Net cash flows from operating activities	841.1	702.7	984.9
Cash flows from (used for) investing activities:
Utility business - Riverside Energy Center	(403.5)	0	0
Utility business - other	(622.0)	(608.1)	(833.3)
Alliant Energy Corporate Services, Inc. and non-regulated businesses	(132.6)	(65.3)	(33.6)
Advances for customer energy efficiency projects	(0.4)	(5.1)	(17.1)
Collections of advances for customer energy efficiency projects	22.9	31.0	34.2
Other	(19.9)	(4.6)	(16.7)
Net cash flows from (used for) investing activities	(1,155.5)	(652.1)	(866.5)
Cash flows from (used for) financing activities:
Common stock dividends	(199.3)	(188.1)	(174.6)
Preferred dividends paid by subsidiaries	(15.9)	(16.8)	(18.7)
Payments to redeem cumulative preferred stock	0	(40.0)	0
Proceeds from issuance of long-term debt	385.0	0.4	500.0
Payments to retire long-term debt	(1.4)	(1.3)	(307.8)
Net change in commercial paper	164.7	55.4	(142.6)
Other	(8.9)	(8.1)	9.3
Net cash flows from (used for) financing activities	324.2	(198.5)	(134.4)
Net increase (decrease) in cash and cash equivalents	9.8	(147.9)	(16.0)
Cash and cash equivalents at beginning of period	11.4	159.3	175.3
Cash and cash equivalents at end of period	21.2	11.4	159.3
Supplemental cash flows information:
Interest, net of capitalized interest	155.2	157.6	165.5
Income taxes, net of refunds	(20.3)	(10.8)	(116.2)
Significant non-cash investing and financing activities:
Accrued capital expenditures	105.3	49.7	75.0
Deferred income tax liabilities [Member]
Other changes in assets and liabilities:
Deferred income taxes	87.6	148.5	28.9
IPL [Member]
Cash flows from operating activities:
Net income	150.2	139.3	143.4
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	188.9	179.1	177.0
Deferred tax expense (benefit) and investment tax credits	19.3	(58.6)	116.0
Equity component of allowance for funds used during construction	(5.2)	(3.5)	(3.0)
Non-cash valuation and regulatory-related charges	3.0	14.8	30.5
Other	7.6	8.7	3.7
Other changes in assets and liabilities:
Accounts receivable	(0.7)	22.4	5.1
Sales of accounts receivable	(10.0)	75.0	65.0
Income tax refunds receivable	13.5	(9.3)	84.3
Production fuel	(7.5)	12.4	21.0
Regulatory assets	(129.0)	(328.8)	(49.5)
Regulatory liabilities	(12.1)	156.3	17.5
Derivative liabilities	(17.5)	9.6	(29.0)
Non-current taxes payable	(2.8)	(4.8)	(28.4)
Pension and other benefit obligations	21.0	(8.3)	(9.0)
Other	(6.3)	16.7	(37.0)
Net cash flows from operating activities	291.0	366.9	549.6
Cash flows from (used for) investing activities:
Utility business - other	(307.5)	(293.7)	(382.8)
Proceeds from sale of wind project assets to affiliate	0	115.3	0
Other	(23.7)	(22.2)	(32.2)
Net cash flows from (used for) investing activities	(331.2)	(200.6)	(415.0)
Cash flows from (used for) financing activities:
Common stock dividends	(122.9)	(73.4)	0
Preferred dividends paid by subsidiaries	(12.6)	(13.5)	(15.4)
Capital contributions from parent	110.0	54.0	50.0
Repayment of capital to parent	0	(100.7)	(118.2)
Payments to redeem cumulative preferred stock	0	(40.0)	0
Proceeds from issuance of long-term debt	0	0	350.0
Payments to retire long-term debt	0	0	(206.3)
Net change in commercial paper	69.2	7.1	(190.0)
Other	(1.1)	(3.4)	0.6
Net cash flows from (used for) financing activities	42.6	(169.9)	(129.3)
Net increase (decrease) in cash and cash equivalents	2.4	(3.6)	5.3
Cash and cash equivalents at beginning of period	2.1	5.7	0.4
Cash and cash equivalents at end of period	4.5	2.1	5.7
Supplemental cash flows information:
Interest, net of capitalized interest	78.3	78.0	82.0
Income taxes, net of refunds	3.3	25.3	(125.9)
Significant non-cash investing and financing activities:
Accrued capital expenditures	53.4	23.9	45.7
IPL [Member] | Deferred income tax liabilities [Member]
Other changes in assets and liabilities:
Deferred income taxes	78.6	145.9	42.0
WPL [Member]
Cash flows from operating activities:
Net income	165.7	163.5	152.3
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	140.9	140.1	108.6
Other amortizations	43.7	42.4	39.5
Deferred tax expense (benefit) and investment tax credits	88.6	93.7	98.5
Equity income from unconsolidated investments, net	(42.1)	(38.7)	(37.8)
Distributions from equity method investments	34.2	32.3	32.2
Equity component of allowance for funds used during construction	(8.9)	(4.1)	(8.2)
Non-cash valuation and regulatory-related charges	(2.9)	10.8	0
Other changes in assets and liabilities:
Accounts receivable	(11.0)	9.8	4.6
Income tax refunds receivable	(2.9)	39.9	40.7
Regulatory assets	(49.1)	(84.3)	28.7
Regulatory liabilities	28.5	12.0	(9.1)
Derivative liabilities	(20.1)	1.1	(23.0)
Non-current taxes payable	(0.1)	(0.3)	(38.5)
Pension and other benefit obligations	31.7	8.8	(2.5)
Other	31.2	1.8	(13.6)
Net cash flows from operating activities	427.4	428.8	372.4
Cash flows from (used for) investing activities:
Utility business - Riverside Energy Center	(403.5)	0	0
Utility business - other	(314.5)	(314.4)	(450.5)
Advances for customer energy efficiency projects	(0.4)	(5.1)	(16.0)
Collections of advances for customer energy efficiency projects	20.9	26.8	30.3
Other	(12.7)	(12.7)	(13.1)
Net cash flows from (used for) investing activities	(710.2)	(305.4)	(449.3)
Cash flows from (used for) financing activities:
Common stock dividends	(112.0)	(112.1)	(109.5)
Preferred dividends paid by subsidiaries	(3.3)	(3.3)	(3.3)
Capital contributions from parent	90.0	25.0	75.0
Repayment of capital to parent	0	0	0
Proceeds from issuance of long-term debt	250.0	0	150.0
Payments to retire long-term debt	0	0	(100.0)
Net change in commercial paper	60.9	(21.7)	47.4
Other	(4.8)	(8.7)	(1.1)
Net cash flows from (used for) financing activities	280.8	(120.8)	58.5
Net increase (decrease) in cash and cash equivalents	(2.0)	2.6	(18.4)
Cash and cash equivalents at beginning of period	2.7	0.1	18.5
Cash and cash equivalents at end of period	0.7	2.7	0.1
Supplemental cash flows information:
Interest, net of capitalized interest	79.5	79.9	80.9
Income taxes, net of refunds	(3.3)	(51.3)	(3.8)
Significant non-cash investing and financing activities:
Accrued capital expenditures	$ 39.5	$ 19.7	$ 27.4