Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net income	$ 1,201.4	$ 1,134.7	$ 1,060.5
Reconciliation to cash provided by operating activities
Depreciation and amortization	975.9	926.3	845.8
Deferred income taxes and ITCs, net	209.4	162.9	297.3
Contributions and payments related to pension and OPEB plans	(113.2)	(65.9)	(77.6)
Equity income in transmission affiliates, net of distributions	(29.1)	(2.9)	(18.6)
Change in -
Accounts receivable and unbilled revenues, net	16.1	98.2	23.5
Materials, supplies, and inventories	21.2	(1.5)	(8.8)
Other current assets	13.4	(7.1)	(10.0)
Accounts payable	(61.3)	1.5	110.6
Other current liabilities	(41.2)	78.7	(67.6)
Other, net	3.4	20.6	290.4
Net cash provided by operating activities	2,196.0	2,345.5	2,445.5
Investing Activities
Capital expenditures	(2,238.8)	(2,260.8)	(2,115.7)
Acquisition of Blooming Grove, net of restricted cash acquired of $24.1	(364.6)	0.0	0.0
Acquisition of Tatanka Ridge	(239.9)	0.0	0.0
Acquisition of Upstream, net of cash and restricted cash acquired of $9.2	0.0	(268.2)	0.0
Acquisition of Bishop Hill III, net of restricted cash acquired of $4.5	0.0	0.0	(162.9)
Acquisition of Forward Wind Energy Center	0.0	0.0	(77.1)
Acquisition of Coyote Ridge	0.0	0.0	(61.4)
Capital contributions to transmission affiliates	(21.2)	(52.6)	(53.5)
Proceeds from the sale of assets and businesses	20.3	37.6	12.1
Proceeds from the sale of investments held in rabbi trust	56.2	0.2	118.6
Purchase of investments held in rabbi trust	(37.8)	0.0	(65.0)
Reimbursement for ATC's construction costs	1.1	32.4	0.0
Insurance proceeds received for property damage	23.2	0.0	0.0
Other, net	(5.3)	16.5	20.5
Net cash used in investing activities	(2,806.8)	(2,494.9)	(2,384.4)
Financing Activities
Exercise of stock options	43.8	67.0	29.1
Purchase of common stock	(99.2)	(140.1)	(72.4)
Dividends paid on common stock	(798.0)	(744.5)	(697.3)
Issuance of long-term debt	2,373.6	1,895.0	1,740.0
Retirement of long-term debt	(1,767.0)	(360.1)	(953.3)
Issuance of short-term loan	340.0	0.0	0.0
Change in other short-term debt	606.1	(609.3)	(4.5)
Payments for debt extinguishment and issuance costs	(55.8)	(12.5)	(9.5)
Purchase of additional ownership interest in Upstream from noncontrolling interest	(31.0)	0.0	0.0
Other, net	(11.4)	(9.9)	(5.7)
Net cash provided by financing activities	601.1	85.6	26.4
Net change in cash, cash equivalents, and restricted cash	(9.7)	(63.8)	87.5
Cash, cash equivalents, and restricted cash at beginning of year	82.3	146.1	58.6
Cash, cash equivalents, and restricted cash at end of year	$ 72.6	$ 82.3	$ 146.1