Revenue - Impact of new revenue guidance on balance sheets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Trade and other receivables	$ 1,524.2	$ 1,463.5
Prepaid expenses and other current assets	484.4	343.4
Total current assets	2,860.8	2,743.3
Other non-current assets	386.3	489.5
Total assets	7,163.4	6,546.0
Accounts payable and accrued expenses	1,145.3	1,047.7
Accrued compensation	888.8	817.9
Total current liabilities	2,322.6	2,038.7
Deferred tax liabilities	110.0	136.4
Other non-current liabilities	352.1	366.6
Total liabilities	5,862.1	5,185.9
Accumulated deficit	(1,297.0)	(1,298.4)
Accumulated other comprehensive loss	(242.8)	(154.4)
Total equity	1,301.3	1,360.1
Total liabilities and shareholders’ equity	$ 7,163.4	6,546.0
Balance Without Adoption of Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Trade and other receivables		1,410.7
Prepaid expenses and other current assets		182.8
Total current assets		2,529.9
Other non-current assets		463.7
Total assets		6,306.8
Accounts payable and accrued expenses		994.9
Accrued compensation		709.8
Total current liabilities		1,877.8
Deferred tax liabilities		119.3
Other non-current liabilities		347.7
Total liabilities		4,989.0
Accumulated deficit		(1,341.2)
Accumulated other comprehensive loss		(153.9)
Total equity		1,317.8
Total liabilities and shareholders’ equity		6,306.8
Accounting Standards Update 2014-09 | Adoption Impact
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Trade and other receivables		52.8
Prepaid expenses and other current assets		160.6
Total current assets		213.4
Other non-current assets		25.8
Total assets		239.2
Accounts payable and accrued expenses		52.8
Accrued compensation		108.1
Total current liabilities		160.9
Deferred tax liabilities		17.1
Other non-current liabilities		18.9
Total liabilities		196.9
Accumulated deficit		42.8
Accumulated other comprehensive loss		(0.5)
Total equity		42.3
Total liabilities and shareholders’ equity		$ 239.2