Earnings per common share - Computation of Earnings Per Common Share Attributable to Unisys Corporation (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic earnings (loss) per common share computation:
Net income (loss) attributable to Unisys Corporation common shareholders	$ (10,800,000)	$ (13,200,000)	$ 26,200,000	$ (19,400,000)	$ 25,000,000.0	$ 6,100,000	$ 3,800,000	$ 40,600,000	$ (17,200,000)	$ 75,500,000	$ (65,300,000)
Weighted average shares (in shares)									55,961	50,946	50,409
Basic earnings (loss) per common share (in dollars per share)	$ (0.17)	$ (0.23)	$ 0.51	$ (0.38)	$ 0.49	$ 0.12	$ 0.07	$ 0.80	$ (0.31)	$ 1.48	$ (1.30)
Diluted earnings (loss) per common share computation:
Net income (loss) attributable to Unisys Corporation common shareholders	$ (10,800,000)	$ (13,200,000)	$ 26,200,000	$ (19,400,000)	$ 25,000,000.0	$ 6,100,000	$ 3,800,000	$ 40,600,000	$ (17,200,000)	$ 75,500,000	$ (65,300,000)
Add interest expense on convertible senior notes, net of tax of zero									0	19,600,000	0
Interest expense on convertible senior notes, tax									0	0	0
Net income (loss) attributable to Unisys Corporation for diluted earnings per share									$ (17,200,000)	$ 95,100,000	$ (65,300,000)
Weighted average shares (in shares)									55,961	50,946	50,409
Plus incremental shares from assumed conversions:
Employee stock plans (in shares)									0	541	0
Convertible senior notes (in shares)									0	21,868	0
Adjusted weighted average shares (in shares)									55,961	73,355	50,409
Diluted earnings (loss) per common share (in dollars per share)	$ (0.17)	$ (0.23)	$ 0.42	$ (0.38)	$ 0.41	$ 0.12	$ 0.07	$ 0.62	$ (0.31)	$ 1.30	$ (1.30)
Stock options and restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive weighted-average securities (in shares)									1,393	1,226	2,206
Common shares issuable upon conversion of the 5.50% convertible senior notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive weighted-average securities (in shares)									16,578	0	21,868
Convertible senior notes interest rate	5.50%								5.50%