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Other Financial Information (Tables)
12 Months Ended
Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]  
Summary of Changes in Operating Working Capital
CHANGES IN WORKING CAPITAL -
Years ended December 31,
(Millions of dollars)202020192018
Accounts receivable$4.9 $(33.4)$86.6 
Inventories(51.7)(6.1)(39.0)
Prepaid expenses and other current assets16.6 (3.3)11.4 
Accounts payable and accrued liabilities8.3 (5.9)(56.7)
Income taxes payable8.8 — — 
Net decrease (increase) in noncash operating working capital$(13.1)$(48.7)$2.3