Segment information - Net premiums written by client location and underwriting location by reportable segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	$ 484.8	$ 469.4
Client location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	241.6	217.9
Client location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	157.7	168.2
Client location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	33.9	30.6
Client location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	51.6	52.7
Underwriting location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	60.9	32.3
Underwriting location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	272.9	277.1
Underwriting location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	133.3	143.0
Underwriting location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	17.7	17.0
Global Property | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	241.3	247.2
Global Property | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	61.9	87.0
Global Property | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	124.8	106.3
Global Property | Client location: Canada, the Caribbean, Bermuda and Latin America | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	24.5	23.2
Global Property | Client location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	30.1	30.7
Global Property | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	8.0	5.3
Global Property | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	154.6	136.5
Global Property | Underwriting location: Canada, the Caribbean, Bermuda and Latin America | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	62.4	89.8
Global Property | Underwriting location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	16.3	15.6
Global A&H | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	134.9	115.5
Global A&H | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	111.1	87.5
Global A&H | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	8.0	11.6
Global A&H | Client location: Canada, the Caribbean, Bermuda and Latin America | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	5.0	4.8
Global A&H | Client location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	10.8	11.6
Global A&H | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	40.3	21.0
Global A&H | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	63.6	68.6
Global A&H | Underwriting location: Canada, the Caribbean, Bermuda and Latin America | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	30.7	25.7
Global A&H | Underwriting location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.3	0.2
Specialty & Casualty | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	108.2	100.6
Specialty & Casualty | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	68.4	37.3
Specialty & Casualty | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	24.8	50.3
Specialty & Casualty | Client location: Canada, the Caribbean, Bermuda and Latin America | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	4.4	2.6
Specialty & Casualty | Client location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	10.6	10.4
Specialty & Casualty | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	12.4	(0.1)
Specialty & Casualty | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	54.6	72.0
Specialty & Casualty | Underwriting location: Canada, the Caribbean, Bermuda and Latin America | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	40.2	27.5
Specialty & Casualty | Underwriting location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	1.0	1.2
Runoff & Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.4	6.1
Runoff & Other | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.2	6.1
Runoff & Other | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.1
Runoff & Other | Client location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.1
Runoff & Other | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.2	$ 6.1
Runoff & Other | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.1
Runoff & Other | Underwriting location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	$ 0.1