Consolidated Statements Of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 467.5	$ 381.7	$ 388.4
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	461.8	411.6	401.3
Other amortizations	21.7	(4.8)	12.4
Deferred tax expense and tax credits	139.6	84.6	114.2
Equity (income) loss from unconsolidated investments, net	(44.8)	(39.6)	(33.8)
Distributions from equity method investments	38.1	28.3	30.6
Equity component of allowance for funds used during construction	(33.6)	(42.3)	(24.4)
Asset valuation charges for Franklin County wind farm	0.0	86.4	0.0
Other	6.7	0.8	15.7
Other changes in assets and liabilities:
Accounts receivable	29.6	(121.4)	36.8
Sales of accounts receivable	(9.0)	16.0	(17.0)
Regulatory assets	(130.8)	(3.6)	(104.5)
Regulatory liabilities	(83.8)	(63.0)	(67.8)
Deferred income taxes	81.7	102.4	94.6
Other	38.7	22.5	24.7
Net cash flows from operating activities	983.4	859.6	871.2
Cash flows from (used for) investing activities:
Utility construction and acquisition expenditures	(1,281.8)	(1,131.2)	(960.3)
Other construction and acquisition expenditures	(185.1)	(65.6)	(74.0)
Proceeds from Minnesota electric and natural gas distribution asset sales	0.0	0.0	139.9
Other	(29.4)	10.3	(24.8)
Net cash flows from (used for) investing activities	(1,496.3)	(1,186.5)	(919.2)
Cash flows from (used for) financing activities:
Common stock dividends	(288.3)	(266.5)	(247.3)
Proceeds from issuance of common stock, net	149.6	26.6	151.2
Proceeds from issuance of long-term debt	550.0	800.0	250.7
Payments to retire long-term debt	(4.6)	(313.4)	(183.0)
Net change in commercial paper and other short-term borrowings	171.1	84.3	18.5
Other	(45.2)	(1.7)	6.8
Net cash flows from (used for) financing activities	532.6	329.3	(3.1)
Net increase (decrease) in cash and cash equivalents	19.7	2.4	(51.1)
Cash and cash equivalents at beginning of period	8.2	5.8	56.9
Cash and cash equivalents at end of period	27.9	8.2	5.8
Supplemental cash flows information:
Interest, net of capitalized interest	(212.6)	(192.4)	(184.8)
Income taxes, net	(11.3)	(9.8)	0.0
Significant non-cash investing and financing activities:
Accrued capital expenditures	196.5	154.4	148.3
IPL [Member]
Cash flows from operating activities:
Net income	227.0	225.8	196.2
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	245.0	210.8	207.2
Deferred tax expense and tax credits	55.8	35.6	28.9
Equity component of allowance for funds used during construction	(21.1)	(35.2)	(18.6)
Other	1.5	2.9	19.5
Other changes in assets and liabilities:
Accounts receivable	(7.9)	(59.7)	20.4
Sales of accounts receivable	(9.0)	16.0	(17.0)
Regulatory assets	(126.2)	(54.7)	(76.3)
Accounts payable	24.0	8.0	(42.7)
Regulatory liabilities	(71.2)	(67.3)	(75.5)
Deferred income taxes	103.7	97.7	82.1
Other	18.4	(18.0)	60.8
Net cash flows from operating activities	440.0	361.9	385.0
Cash flows from (used for) investing activities:
Utility construction and acquisition expenditures	(676.0)	(689.7)	(619.3)
Proceeds from Minnesota electric and natural gas distribution asset sales	0.0	0.0	139.9
Other	(30.4)	(3.9)	(32.5)
Net cash flows from (used for) investing activities	(706.4)	(693.6)	(511.9)
Cash flows from (used for) financing activities:
Common stock dividends	(156.1)	(151.9)	(140.0)
Capital contributions from parent	200.0	190.0	165.0
Proceeds from issuance of long-term debt	250.0	300.0	250.0
Payments to retire long-term debt	0.0	0.0	(150.0)
Other	(27.2)	(7.6)	1.1
Net cash flows from (used for) financing activities	266.7	330.5	126.1
Net increase (decrease) in cash and cash equivalents	0.3	(1.2)	(0.8)
Cash and cash equivalents at beginning of period	3.3	4.5	5.3
Cash and cash equivalents at end of period	3.6	3.3	4.5
Supplemental cash flows information:
Interest, net of capitalized interest	(111.8)	(99.7)	(93.9)
Income taxes, net	8.6	(11.1)	19.3
Significant non-cash investing and financing activities:
Accrued capital expenditures	76.4	53.8	77.0
WPL [Member]
Cash flows from operating activities:
Net income	186.6	192.8	177.6
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	212.9	192.5	184.3
Other amortizations	17.7	(8.7)	5.3
Deferred tax expense and tax credits	53.9	114.5	77.6
Equity (income) loss from unconsolidated investments, net	(0.7)	(39.8)	(35.1)
Other	(13.3)	(17.8)	(10.2)
Other changes in assets and liabilities:
Accounts receivable	17.7	(47.6)	3.7
Regulatory assets	(4.7)	51.1	(28.2)
Other	(5.1)	44.6	39.7
Net cash flows from operating activities	465.7	521.4	449.8
Cash flows from (used for) investing activities:
Utility construction and acquisition expenditures	(637.4)	(453.0)	(344.3)
Other	(28.3)	(25.9)	(13.9)
Net cash flows from (used for) investing activities	(665.7)	(478.9)	(358.2)
Cash flows from (used for) financing activities:
Common stock dividends	(125.9)	(135.0)	(126.9)
Capital contributions from parent	90.0	60.0	0.0
Proceeds from issuance of long-term debt	300.0	0.0	0.0
Net change in commercial paper and other short-term borrowings	(27.3)	32.4	19.9
Other	(17.9)	3.9	(30.9)
Net cash flows from (used for) financing activities	218.9	(38.7)	(137.9)
Net increase (decrease) in cash and cash equivalents	18.9	3.8	(46.3)
Cash and cash equivalents at beginning of period	4.2	0.4	46.7
Cash and cash equivalents at end of period	23.1	4.2	0.4
Supplemental cash flows information:
Interest, net of capitalized interest	(91.7)	(91.5)	(93.1)
Income taxes, net	(8.4)	27.8	(7.4)
Significant non-cash investing and financing activities:
Accrued capital expenditures	114.5	93.1	55.2
Transfer of investment in ATC and tax liability to ATI	$ 0.0	$ (163.6)	$ 0.0