Geographic Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of operating segments | segment	1
Total operating revenues	$ 6,145.6	$ 6,117.4	$ 5,314.1
Long-lived assets	563.8	583.5	468.7
Americas
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	4,541.6	4,290.2	3,245.0
Long-lived assets	384.0	416.0	319.4
UK
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	632.8	819.7	977.2
Long-lived assets	147.6	138.5	122.8
EMEA Ex UK
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	645.1	698.3	815.9
Long-lived assets	9.1	8.2	8.1
Asia
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	326.1	309.2	276.0
Long-lived assets	$ 23.1	$ 20.8	$ 18.4