Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Millions	3 Months Ended				6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 208.1		$ 171.4				$ 434.5	$ 371.2
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization							376.4	342.7
Deferred tax expense and tax credits							62.5	102.7
Equity income from unconsolidated investments, net	(9.8)		(10.1)				(41.6)	(32.9)
Other							8.2	25.8
Other changes in assets and liabilities:
Accounts receivable							(325.2)	(268.3)
Regulatory assets							27.4	(108.9)
Accounts payable							(47.3)	3.8
Regulatory liabilities							14.7	(64.8)
Deferred income taxes							32.7	101.0
Other							(100.1)	(21.0)
Net cash flows from operating activities		$ 156.3		$ 173.5	$ 274.4	$ 249.8	442.2	451.3
Cash flows used for investing activities:
Utility business construction and acquisition expenditures							(1,080.2)	(892.5)
Other construction and acquisition expenditures							(47.8)	(156.9)
Cash receipts on sold receivables							337.2	432.1
Other							(24.9)	(21.8)
Net cash flows used for investing activities							(815.7)	(639.1)
Cash flows from financing activities:
Common stock dividends							(233.3)	(215.7)
Proceeds from issuance of common stock, net							191.3	143.2
Proceeds from issuance of long-term debt							1,500.0	0.0
Payments to retire long-term debt							(603.1)	(2.5)
Net change in commercial paper and other short-term borrowings							(278.4)	281.2
Other							10.9	(16.3)
Net cash flows from financing activities							587.4	189.9
Net increase (decrease) in cash, cash equivalents and restricted cash							213.9	2.1
Cash, cash equivalents and restricted cash at beginning of period		33.9		13.1	33.9	13.1	33.9	13.1	$ 13.1
Cash, cash equivalents and restricted cash at end of period	247.8		15.2				247.8	15.2	33.9
Supplemental cash flows information:
Interest, net of capitalized interest							(171.6)	(158.5)
Income taxes, net							(5.0)	(11.4)
Significant non-cash investing and financing activities:
Accrued capital expenditures							236.2	197.2
Beneficial interest obtained in exchange for securitized accounts receivable							243.7	115.3
IPL [Member]
Cash flows from operating activities:
Net income	129.1		123.0				232.6	208.1
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization							209.2	181.0
Other							(18.6)	26.2
Other changes in assets and liabilities:
Accounts receivable							(353.2)	(328.7)
Regulatory assets							14.0	(107.8)
Accounts payable							(34.5)	11.6
Regulatory liabilities							7.5	(49.6)
Deferred income taxes							22.9	88.9
Other							(41.0)	8.7
Net cash flows from operating activities		1.8		16.1	12.8	15.8	38.9	38.4
Cash flows used for investing activities:
Utility business construction and acquisition expenditures							(635.2)	(470.1)
Cash receipts on sold receivables							337.2	432.1
Other							(30.9)	(21.6)
Net cash flows used for investing activities							(328.9)	(59.6)
Cash flows from financing activities:
Common stock dividends							(125.9)	(117.0)
Capital contributions from parent							230.0	100.0
Proceeds from issuance of long-term debt							500.0	0.0
Payments to retire long-term debt							(100.0)	0.0
Net change in commercial paper and other short-term borrowings							0.0	44.0
Other							15.0	(2.6)
Net cash flows from financing activities							519.1	24.4
Net increase (decrease) in cash, cash equivalents and restricted cash							229.1	3.2
Cash, cash equivalents and restricted cash at beginning of period		7.2		4.2	7.2	4.2	7.2	4.2	4.2
Cash, cash equivalents and restricted cash at end of period	236.3		7.4				236.3	7.4	7.2
Supplemental cash flows information:
Interest, net of capitalized interest							(89.1)	(84.1)
Income taxes, net							(2.4)	13.2
Significant non-cash investing and financing activities:
Accrued capital expenditures							142.4	71.0
Beneficial interest obtained in exchange for securitized accounts receivable							243.7	115.3
WPL [Member]
Cash flows from operating activities:
Net income							170.1	133.4
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization							164.2	158.8
Deferred tax expense and tax credits							50.3	60.1
Other							(12.0)	4.8
Other changes in assets and liabilities:
Accounts receivable							29.7	41.8
Regulatory assets							13.4	(1.1)
Other							(70.2)	(36.6)
Net cash flows from operating activities							345.5	361.2
Cash flows used for investing activities:
Utility business construction and acquisition expenditures							(445.0)	(454.0)
Other							(23.7)	(18.6)
Net cash flows used for investing activities							(468.7)	(472.6)
Cash flows from financing activities:
Common stock dividends							(105.0)	(94.5)
Capital contributions from parent							200.0	40.0
Net change in commercial paper and other short-term borrowings							13.4	172.3
Other							(1.7)	(9.8)
Net cash flows from financing activities							106.7	108.0
Net increase (decrease) in cash, cash equivalents and restricted cash							(16.5)	(3.4)
Cash, cash equivalents and restricted cash at beginning of period		$ 24.2		$ 6.9	$ 24.2	$ 6.9	24.2	6.9	6.9
Cash, cash equivalents and restricted cash at end of period	$ 7.7		$ 3.5				7.7	3.5	$ 24.2
Supplemental cash flows information:
Interest, net of capitalized interest							(68.5)	(68.1)
Income taxes, net							(11.2)	(20.2)
Significant non-cash investing and financing activities:
Accrued capital expenditures							$ 86.2	$ 122.3