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INCOME TAXES (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Effective Income Tax Rate Reconciliation, Amount        
Statutory federal income tax, amount $ 65.9 $ 51.5 $ 242.0 $ 206.1
State income taxes net of federal tax benefit, amount 19.7 20.3 72.1 66.8
Federal excess deferred tax amortization - Wisconsin unprotected, amount (12.5) 0.0 (45.7) 0.0
Wind production tax credits, amount (11.2) (6.8) (39.0) (26.4)
Federal excess deferred tax amortization, amount (8.3) (12.0) (30.2) (32.7)
Uncertain tax positions, amount (5.1) 0.3 (5.8) (2.6)
Excess tax benefits-stock options, amount (0.6) (3.9) (6.9) (15.5)
Tax repairs, amount 0.9 (30.7) 2.4 (90.7)
Other, amount (1.9) (7.4) 1.8 (13.5)
Total income tax expense, amount $ 46.9 $ 11.3 $ 190.7 $ 91.5
Effective Income Tax Rate Reconciliation, Percent        
Statutory federal income tax, percentage 21.00% 21.00% 21.00% 21.00%
State income taxes net of federal tax benefit, percentage 6.30% 8.20% 6.30% 6.80%
Federal excess deferred tax amortization - Wisconsin unprotected, percentage (4.00%) 0.00% (4.00%) 0.00%
Wind production tax credits, percentage (3.60%) (2.80%) (3.40%) (2.70%)
Federal excess deferred tax amortization, percentage (2.60%) (4.90%) (2.60%) (3.30%)
Excess tax benefits-stock options, percentage (0.20%) (1.60%) (0.60%) (1.60%)
Uncertain tax position, percentage (1.60%) 0.10% (0.50%) (0.30%)
Tax repairs, percentage 0.30% (12.50%) 0.20% (9.20%)
Other, percentage (0.70%) (2.90%) 0.10% (1.40%)
Total income tax expense, percentage 14.90% 4.60% 16.50% 9.30%