Segment and Geographical Area Information (External Revenue and Long-Lived Assets) (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
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 332.4	$ 313.8	$ 273.9	$ 258.9	$ 262.7	$ 261.3	$ 252.4	$ 243.5	$ 1,179.0	$ 1,019.9	$ 911.7
Long-lived assets	154.7				143.5				154.7	143.5
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									866.4	764.2	687.0
Long-lived assets	131.2				126.4				131.2	126.4
UNITED KINGDOM
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									93.9	72.4	64.7
Long-lived assets	6.9				3.8				6.9	3.8
Europe excluding the United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									88.0	81.2	69.9
Long-lived assets	2.3				1.3				2.3	1.3
AUSTRALIA
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									39.5	40.9	34.6
Long-lived assets	4.2				5.0				4.2	5.0
CANADA
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									56.9	30.7	29.4
Long-lived assets	2.9				0.3				2.9	0.3
Asia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									27.9	24.5	21.2
Long-lived assets	6.6				6.5				6.6	6.5
Segment, Geographical, Group of Other Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									6.4	6.0	4.9
Long-lived assets	0.6				0.2				0.6	0.2
Non United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									312.6	255.7	$ 224.7
Long-lived assets	$ 23.5				$ 17.1				$ 23.5	$ 17.1