Segment and Geographical Area Information (External Revenue and Long-Lived Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 2,275.1	$ 2,038.6	$ 1,870.6
Long-lived assets	218.9	207.7
Operating lease assets	181.2	163.9
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	1,638.8	1,470.6	1,353.9
Long-lived assets	189.5	178.5
Operating lease assets	92.9	100.7
UNITED KINGDOM
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	167.4	148.0	133.6
Long-lived assets	6.1	7.2
Operating lease assets	14.7	16.9
Europe excluding the United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	203.8	185.5	162.9
Long-lived assets	5.3	6.5
Operating lease assets	19.1	18.1
AUSTRALIA
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	62.4	58.4	55.8
Long-lived assets	1.6	1.9
Operating lease assets	2.4	3.2
CANADA
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	140.4	116.3	109.8
Long-lived assets	6.6	3.6
Operating lease assets	7.7	8.2
Asia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	49.6	49.3	44.8
Long-lived assets	9.6	9.9
Operating lease assets	44.2	16.5
Segment, Geographical, Group of Other Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	12.7	10.5	9.8
Long-lived assets	0.2	0.1
Operating lease assets	0.2	0.3
Non United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	636.3	568.0	$ 516.7
Long-lived assets	29.4	29.2
Operating lease assets	$ 88.3	$ 63.2