Revenue - Narrative (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract liabilities	$ 54.4				$ 66.8				$ 54.4	$ 66.8
Contract liabilities reduced due to revenue recognition criteria being satisfied									465.4	587.3
Total liabilities and shareholders' equity	7,163.4				6,546.0				7,163.4	6,546.0
Revenue	2,607.5	$ 2,118.8	$ 2,121.7	$ 1,903.0	2,401.9	$ 2,076.0	$ 1,974.3	$ 1,767.7	8,751.0	8,219.9	$ 6,923.9
Total assets	7,163.4				6,546.0				7,163.4	6,546.0
Prepaid expenses and other current assets	484.4				343.4				484.4	343.4
Other non-current assets	386.3				489.5				386.3	489.5
Trade and other receivables	1,524.2				1,463.5				1,524.2	1,463.5
Total liabilities	5,862.1				5,185.9				5,862.1	5,185.9
Accrued compensation	888.8				817.9				888.8	817.9
Trade and other receivables, net of allowance balance of $58.4 million and $49.5 million, as of December 31, 2019 and 2018, respectively	1,524.2				1,463.5				1,524.2	1,463.5
Deferred tax liabilities	110.0				136.4				110.0	136.4
Other non-current liabilities	352.1				366.6				352.1	366.6
Net loss	3.1	$ 11.7	$ 6.3	$ (20.9)	(18.0)	$ (41.4)	$ (33.5)	$ (92.9)	0.2	(185.8)	(221.3)
Prepaid expenses and other current assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract assets	313.4				160.6				313.4	160.6
Other non-current assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prepaid expenses and other current assets	$ 26.7				25.8				26.7	25.8
Other current liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract liabilities					18.9					18.9
Adoption Impact | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total liabilities and shareholders' equity					239.2					239.2
Revenue										432.8
Total assets					239.2					239.2
Prepaid expenses and other current assets					160.6					160.6
Other non-current assets					25.8					25.8
Trade and other receivables					52.8					52.8
Total liabilities					196.9					196.9
Accrued compensation					108.1					108.1
Trade and other receivables, net of allowance balance of $58.4 million and $49.5 million, as of December 31, 2019 and 2018, respectively					52.8					52.8
Deferred tax liabilities					17.1					17.1
Other non-current liabilities					18.9					18.9
Net loss										6.4
Balance Without Adoption of Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total liabilities and shareholders' equity					6,306.8					6,306.8
Revenue										7,787.1
Total assets					6,306.8					6,306.8
Prepaid expenses and other current assets					182.8					182.8
Other non-current assets					463.7					463.7
Trade and other receivables					1,410.7					1,410.7
Total liabilities					4,989.0					4,989.0
Accrued compensation					709.8					709.8
Trade and other receivables, net of allowance balance of $58.4 million and $49.5 million, as of December 31, 2019 and 2018, respectively					1,410.7					1,410.7
Deferred tax liabilities					119.3					119.3
Other non-current liabilities					$ 347.7					347.7
Net loss										(192.2)
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amounts reclassified from AOCI to the statement of operations												$ 1.2
Net loss									$ 0.2	$ (185.8)	$ (221.3)
Accumulated Deficit | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amounts reclassified from AOCI to the statement of operations													$ 35.9
Accumulated Deficit | Adoption Impact | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amounts reclassified from AOCI to the statement of operations													$ 35.9