Revenues - Disaggregated Revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 625.0	$ 581.2
U.S.
Disaggregation of Revenue [Line Items]
Revenues	480.6	449.5
U.K.
Disaggregation of Revenue [Line Items]
Revenues	44.2	35.0
Other countries
Disaggregation of Revenue [Line Items]
Revenues	100.2	96.7
Insurance
Disaggregation of Revenue [Line Items]
Revenues	451.2	412.6
Insurance | Underwriting & rating
Disaggregation of Revenue [Line Items]
Revenues	303.5	280.6
Insurance | Claims
Disaggregation of Revenue [Line Items]
Revenues	147.7	132.0
Energy and Specialized Markets
Disaggregation of Revenue [Line Items]
Revenues	130.8	125.5
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	$ 43.0	$ 43.1