Recent accounting pronouncements and accounting changes (Detail) - USD ($) $ in Millions			3 Months Ended								12 Months Ended
	Jul. 01, 2018	Jan. 01, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Revenue
Revenue			$ 760.9	$ 688.3	$ 667.4	$ 708.4	$ 744.8	$ 666.3	$ 666.2	$ 664.5	$ 2,825.0	$ 2,741.8		$ 2,820.7
Cost of revenue:
Cost of revenue											2,138.7	2,194.1	[1]	2,203.3	[1]
Selling, general and administrative expenses											370.3	411.9	[1]	441.2	[1]
Operating income											284.1	97.1	[1]	129.2	[1]
Income before income taxes			38.1	22.2	20.3	62.6	27.4	(40.4)	(42.3)	(16.8)	143.2	(72.1)		20.5
Provision for income taxes											64.3	(5.5)		57.2
Consolidated net income (loss)											78.9	(66.6)		(36.7)
Net income attributable to Unisys Corporation			25.0	$ 6.1	$ 3.8	40.6	50.5	$ (41.1)	$ (42.0)	$ (32.7)	75.5	(65.3)		(47.7)
Assets
Accounts receivable, net current			509.2				503.3				509.2	503.3
Contract assets			29.7				0.0				29.7	0.0
Inventories			27.3								27.3
Prepaid expenses and other current assets			130.2				126.2				130.2	126.2
Deferred income taxes - long-term			109.3				119.9				109.3	119.9		146.1
Other long-term assets			202.6				190.6				202.6	190.6
Total assets			2,457.6				2,542.4				2,457.6	2,542.4		2,021.6
Liabilities
Deferred revenue - current			294.4				327.5				294.4	327.5
Other accrued liabilities - current			350.0				391.5				350.0	391.5
Long-term deferred revenue			157.2				159.0				157.2	159.0
Other long-term liabilities			77.4				100.0				77.4	100.0
Equity
Accumulated deficit			(1,694.0)				(1,963.1)				(1,694.0)	(1,963.1)
Total liabilities and deficit			2,457.6				2,542.4				2,457.6	2,542.4
Reclassification pursuant to ASU No. 2018-02											0.0
AOCI
Equity
Reclassification pursuant to ASU No. 2018-02	$ (208.7)										(208.7)
Accumulated Deficit
Cost of revenue:
Consolidated net income (loss)											75.5	(65.3)		(47.7)
Equity
Reclassification pursuant to ASU No. 2018-02	$ 208.7										208.7
Services
Revenue
Revenue											2,386.3	2,328.2		2,406.3
Cost of revenue:
Cost of revenue											2,010.5	2,033.8	[1]	2,035.2	[1]
Operating income											67.6	64.8		46.9
Assets
Total assets			1,013.1				985.9				1,013.1	985.9		963.3
Technology
Revenue
Revenue											438.7	413.6		414.4
Cost of revenue:
Cost of revenue											128.2	160.3	[1]	168.1	[1]
Operating income											237.8	170.6		161.5
Assets
Total assets			423.5				$ 378.6				423.5	378.6		375.7
Technology | Software License Extensions and Renewals
Revenue
Revenue		$ 53.0				$ 53.0
Balances Without Adoption of Topic 606
Cost of revenue:
Selling, general and administrative expenses											371.2
Operating income											145.9
Income before income taxes											5.0
Provision for income taxes											45.7
Consolidated net income (loss)											(40.7)
Net income attributable to Unisys Corporation											(44.1)
Assets
Accounts receivable, net current			520.1								520.1
Contract assets			0.0								0.0
Inventories			37.9								37.9
Prepaid expenses and other current assets			131.7								131.7
Deferred income taxes - long-term			111.0								111.0
Other long-term assets			174.8								174.8
Total assets			2,424.8								2,424.8
Liabilities
Deferred revenue - current			364.7								364.7
Other accrued liabilities - current			329.3								329.3
Long-term deferred revenue			167.6								167.6
Other long-term liabilities			82.9								82.9
Equity
Accumulated deficit			(1,792.3)								(1,792.3)
Total liabilities and deficit			2,424.8								2,424.8
Balances Without Adoption of Topic 606 | Services
Revenue
Revenue											2,380.1
Cost of revenue:
Cost of revenue											2,007.5
Balances Without Adoption of Topic 606 | Technology
Revenue
Revenue											298.8
Cost of revenue:
Cost of revenue											122.4
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit cost, other than service cost												$ 92.5		$ 81.6
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment		(21.4)
Accounting Standards Update 2014-09 | Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment		(21.4)
Accounting Standards Update 2014-09 | Adjustments
Cost of revenue:
Selling, general and administrative expenses											0.9
Operating income											(138.2)
Income before income taxes											(138.2)
Provision for income taxes											(18.6)
Consolidated net income (loss)											(119.6)
Net income attributable to Unisys Corporation											(119.6)
Assets
Accounts receivable, net current			10.9								10.9
Contract assets			(29.7)								(29.7)
Inventories			10.6								10.6
Prepaid expenses and other current assets			1.5								1.5
Deferred income taxes - long-term			1.7								1.7
Other long-term assets			(27.8)								(27.8)
Total assets			(32.8)								(32.8)
Liabilities
Deferred revenue - current			70.3								70.3
Other accrued liabilities - current			(20.7)								(20.7)
Long-term deferred revenue			10.4								10.4
Other long-term liabilities			5.5								5.5
Equity
Accumulated deficit			(98.3)								(98.3)
Total liabilities and deficit			$ (32.8)								(32.8)
Accounting Standards Update 2014-09 | Adjustments | Services
Revenue
Revenue											(6.2)
Cost of revenue:
Cost of revenue											(3.0)
Accounting Standards Update 2014-09 | Adjustments | Technology
Revenue
Revenue											(139.9)
Cost of revenue:
Cost of revenue											$ (5.8)
Accounting Standards Update 2017-05
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment		6.3
Accounting Standards Update 2017-05 | Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment		$ 6.3

[1] Amounts were changed to conform to the current-year presentation. See Note 2.