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Schedule II, Valuation and Qualifying Accounts
 12 Months Ended
Dec. 31, 2018
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]  
Schedule II, Valuation and Qualifying Accounts
NSP-Wisconsin and Subsidiaries Valuation and Qualifying Accounts Years Ended Dec. 31
 
 
Allowance for bad debts
(Millions of Dollars)
 
2019
 
2018
 
2017
Balance at Jan. 1
 
$
5.6

 
$
4.9

 
$
4.9

Additions charged to costs and expenses
 
7.0

 
4.2

 
4.1

Additions charged to other accounts (a)
 
2.1

 
1.0

 
0.9

Deductions from reserves (b)
 
(9.2
)
 
(4.5
)
 
(5.0
)
Balance at Dec. 31
 
$
5.5

 
$
5.6

 
$
4.9

(a) 
Recovery of amounts previously written off.
(b) 
Deductions related primarily to bad debt write-offs.