Revenue - Impact of new revenue guidance on balance sheets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Trade and other receivables	$ 1,299.9	$ 1,314.0
Prepaid expenses and other current assets	373.9	176.3
Total current assets	2,623.8	1,910.5
Other non-current assets	500.0	432.8
Total assets	6,432.4	5,797.9
Accounts payable and accrued expenses	748.4	771.2
Accrued compensation	874.9	864.8
Total current liabilities	1,900.9	1,965.6
Deferred tax liabilities	107.9	157.5
Other non-current liabilities	368.0	386.9
Total liabilities	5,023.3	5,294.0
Accumulated deficit	(1,302.2)	(1,165.2)
Accumulated other comprehensive loss	(98.8)	(87.2)
Total equity	1,409.1	503.9	$ 430.9	$ 590.0
Total liabilities and shareholders’ equity	6,432.4	$ 5,797.9
Balance Without Adoption of Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Trade and other receivables	1,246.7
Prepaid expenses and other current assets	189.0
Total current assets	2,385.7
Other non-current assets	474.2
Total assets	6,168.5
Accounts payable and accrued expenses	697.7
Accrued compensation	748.2
Total current liabilities	1,723.5
Deferred tax liabilities	92.0
Other non-current liabilities	349.0
Total liabilities	4,811.0
Accumulated deficit	(1,354.0)
Accumulated other comprehensive loss	(98.6)
Total equity	1,357.5
Total liabilities and shareholders’ equity	6,168.5
Accounting Standards Update 2014-09 | Adoption Impact
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Trade and other receivables	53.2
Prepaid expenses and other current assets	184.9
Total current assets	238.1
Other non-current assets	25.8
Total assets	263.9
Accounts payable and accrued expenses	50.7
Accrued compensation	126.7
Total current liabilities	177.4
Deferred tax liabilities	15.9
Other non-current liabilities	19.0
Total liabilities	212.3
Accumulated deficit	51.8
Accumulated other comprehensive loss	(0.2)
Total equity	51.6
Total liabilities and shareholders’ equity	$ 263.9