Condensed Consolidated Statements Of Cash Flows (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities:
Net income	$ 99.2	$ 110.6
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	100.2	95.5
Deferred taxes and investment tax credits	29.7	35.9
Other	(0.1)	5.9
Other changes in assets and liabilities:
Accounts receivable	6.6	(55.6)
Sales of accounts receivable	52.0	46.0
Production fuel	16.2	34.8
Gas stored underground	49.5	28.5
Accounts payable	(32.8)	25.4
Other	(5.8)	(33.4)
Net cash flows from operating activities	314.7	293.6
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(226.0)	(159.0)
Alliant Energy Corporate Services, Inc. and non-regulated businesses construction and acquisition expenditures	(19.8)	(14.0)
Other	(5.1)	(3.8)
Net cash flows used for investing activities	(250.9)	(176.8)
Cash flows used for financing activities:
Common stock dividends	(60.7)	(56.5)
Proceeds from issuance of common stock, net	122.1	0
Net change in commercial paper	(99.8)	(58.1)
Other	15.3	2.5
Net cash flows used for financing activities	(23.1)	(112.1)
Net increase (decrease) in cash and cash equivalents	40.7	4.7
Cash and cash equivalents at beginning of period	56.9	9.8
Cash and cash equivalents at end of period	97.6	14.5
Supplemental cash flows information:
Interest, net of capitalized interest	(40.2)	(41.4)
Income taxes, net	0.1	3.9
Significant non-cash investing and financing activities:
Accrued capital expenditures	119.0	85.9
IPL [Member]
Cash flows from operating activities:
Net income	50.4	46.0
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	51.8	48.7
Other	12.5	10.1
Other changes in assets and liabilities:
Accounts receivable	(2.2)	(48.9)
Sales of accounts receivable	52.0	46.0
Gas stored underground	26.4	13.3
Accounts payable	(36.1)	28.8
Accounts payable to associated companies	3.1	(28.8)
Other	12.2	(9.6)
Net cash flows from operating activities	170.1	105.6
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(150.6)	(91.1)
Other	(5.7)	(5.5)
Net cash flows used for investing activities	(156.3)	(96.6)
Cash flows used for financing activities:
Common stock dividends	(35.0)	(35.0)
Capital contributions from parent	0	30.0
Other	18.5	(2.6)
Net cash flows used for financing activities	(16.5)	(7.6)
Net increase (decrease) in cash and cash equivalents	(2.7)	1.4
Cash and cash equivalents at beginning of period	5.3	4.4
Cash and cash equivalents at end of period	2.6	5.8
Supplemental cash flows information:
Interest, net of capitalized interest	(18.4)	(19.3)
Income taxes, net	6.2	(1.5)
Significant non-cash investing and financing activities:
Accrued capital expenditures	82.8	46.3
WPL [Member]
Cash flows from operating activities:
Net income	44.8	54.8
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	46.0	44.7
Deferred taxes and investment tax credits	10.4	20.8
Other	(0.9)	8.7
Other changes in assets and liabilities:
Derivative assets	0.2	(19.8)
Gas stored underground	23.1	15.2
Regulatory liabilities	5.1	26.1
Other	29.1	(21.8)
Net cash flows from operating activities	157.8	128.7
Cash flows used for investing activities:
Utility construction and acquisition expenditures	(75.4)	(67.9)
Other	(3.0)	(2.6)
Net cash flows used for investing activities	(78.4)	(70.5)
Cash flows used for financing activities:
Common stock dividends	(31.8)	(29.7)
Net change in commercial paper	0	(28.2)
Other	(3.4)	4.1
Net cash flows used for financing activities	(35.2)	(53.8)
Net increase (decrease) in cash and cash equivalents	44.2	4.4
Cash and cash equivalents at beginning of period	46.7	0.5
Cash and cash equivalents at end of period	90.9	4.9
Supplemental cash flows information:
Interest, net of capitalized interest	(22.6)	(23.2)
Income taxes, net	9.1	0.6
Significant non-cash investing and financing activities:
Accrued capital expenditures	$ 32.9	$ 33.7