CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) shares in Millions, $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Cash and cash equivalents	$ 386.5	$ 274.4
Accounts receivable, net	646.9	678.7
Prepaid expenses and income taxes	20.2	23.3
Other current assets	20.6	18.0
Total current assets	1,074.2	994.4
Property and equipment, net	68.3	69.4
Operating lease right-of-use assets	76.2	84.9
Identifiable intangible assets, net	475.6	487.9
Goodwill	1,617.5	1,618.4
Other	22.7	23.0
Total assets	3,334.5	3,278.0
Accounts payable	29.9	39.8
Accrued payroll and contract professional pay	284.7	266.1
Operating lease liabilities	28.0	29.4
Other current liabilities	86.3	80.9
Total current liabilities	428.9	416.2
Long-term Debt	1,033.7	1,033.4
Operating Lease, Liability, Noncurrent	55.0	62.9
Deferred income tax liabilities	108.6	108.7
Other	62.5	69.7
Total liabilities	1,688.7	1,690.9
Commitments and Contingencies (Note 5)
Preferred Stock, Value, Issued	0.0	0.0
Common Stock, Value, Issued	0.5	0.5
Paid-in capital	674.5	661.3
Retained earnings	975.0	926.3
Accumulated other comprehensive loss	(4.2)	(1.0)
Total stockholders’ equity	1,645.8	1,587.1
Total liabilities and stockholders’ equity	$ 3,334.5	$ 3,278.0
Common Stock, Shares Outstanding	53.2
Common Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common Stock, shares authorized (in shares)	75.0	75.0
Preferred Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred Stock, shares authorized (in shares)	1.0	1.0
Preferred Stock, shares issued (in shares)	0.0	0.0