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Earnings per common share - Computation of Earnings Per Common Share Attributable to Unisys Corporation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic earnings (loss) per common share computation:
Net loss attributable to Unisys Corporation common shareholders									$ (65.3)	$ (47.7)	$ (109.9)
Weighted average shares (in shares)									50,409	50,060	49,905
Basic loss per common share (in dollars per share)	$ 1.00	$ (0.81)	$ (0.83)	$ (0.65)	$ (0.02)	$ (0.56)	$ 0.43	$ (0.80)	$ (1.30)	$ (0.95)	$ (2.20)
Diluted earnings (loss) per common share computation:
Net loss attributable to Unisys Corporation for diluted earnings per share									$ (65.3)	$ (47.7)	$ (109.9)
Weighted average shares (in shares)									50,409	50,060	49,905
Diluted loss per common share (in dollars per share)	$ 0.76	$ (0.81)	$ (0.83)	$ (0.65)	$ (0.02)	$ (0.56)	$ 0.36	$ (0.80)	$ (1.30)	$ (0.95)	$ (2.20)
Stock options and restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive weighted-average securities (in shares)									2,206	3,553	2,915
5.50% convertible senior notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive weighted-average securities (in shares)									21,868	17,230	0
Convertible senior notes interest rate	5.50%								5.50%