Revenue - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	$ 2,076.0	$ 1,709.3	$ 5,818.0	$ 4,871.2
Contract assets	210.7		210.7			$ 0.0
Total assets	6,432.4		6,432.4			5,797.9
Prepaid expenses and other current assets	373.9		373.9			176.3
Other non-current assets	500.0		500.0			432.8
Trade and other receivables	1,299.9		1,299.9			1,314.0
Total liabilities	5,023.3		5,023.3			5,294.0
Accrued compensation	874.9		874.9			864.8
Deferred tax liabilities	107.9		107.9			157.5
Other non-current liabilities	368.0		368.0			$ 386.9
Net loss	(48.7)	$ (78.6)	(172.9)	(245.6)
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new revenue accounting standard					$ 35.9
Prepaid expenses and other current assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract assets	184.9		184.9
Other non-current assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract assets	25.8		25.8
Adoption Impact | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	123.0		351.4
Total assets	263.9		263.9
Prepaid expenses and other current assets	184.9		184.9
Other non-current assets	25.8		25.8
Trade and other receivables	53.2		53.2
Total liabilities	212.3		212.3
Accrued compensation	126.7		126.7
Deferred tax liabilities	15.9		15.9
Other non-current liabilities	19.0		19.0
Net loss	7.4		15.9
Adoption Impact | Accounting Standards Update 2014-09 | Reimbursed expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	98.5		302.0
Adoption Impact | Accounting Standards Update 2014-09 | Variable consideration
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	$ 24.5		49.4
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net loss			$ (172.9)	$ (245.6)
Accumulated Deficit | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new revenue accounting standard					35.9
Accumulated Deficit | Adoption Impact | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new revenue accounting standard					$ 35.9