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Schedule II - Valuation and Qualifying Accounts
 12 Months Ended
Dec. 31, 2020
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]  
Schedule II - Valuation and Qualifying Accounts
SCHEDULE II—VALUATION AND QUALIFYING ACCOUNTS
(in millions)
Balance at
beginning of
period		Additions Charged to
Costs and
Expenses		Deductions Balance at
end of
period
Year Ended December 31, 2020
Valuation allowance for deferred tax assets$31 $38 $— $69 
Allowance for obsolete inventory parts22 5 — 27 
Allowance for doubtful accounts1 6 5 
(1)
2 
Total$54 $49 $5 $98 
Year Ended December 31, 2019
Valuation allowance for deferred tax assets$21 $10 $— $31 
Allowance for obsolete inventory parts18 4 — 22 
Allowance for doubtful accounts1 6 6 
(1)
1 
Total$40 $20 $6 $54 
Year Ended December 31, 2018
Valuation allowance for deferred tax assets$1 $20 $— $21 
Allowance for obsolete inventory parts14 4 — 18 
Allowance for doubtful accounts1 6 6 
(1)
1 
Total$16 $30 $6 $40 
 
(1)Uncollectible accounts written off, net of recoveries.