Segment information - Net premiums written by client location and underwriting location by reportable segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	$ 1,357.1	$ 1,090.2	$ 938.1
Client location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	794.7	563.1	463.0
Client location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	276.5	262.3	258.9
Client location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	103.6	111.4	88.3
Client location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	182.3	153.4	127.9
Underwriting location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	180.1	150.5	319.3
Underwriting location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	692.1	583.3	531.3
Underwriting location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	433.8	315.5	51.5
Underwriting location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	51.1	40.9	36.0
Global Property | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	655.0	556.2	514.2
Global Property | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	284.4	220.8	214.8
Global Property | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	171.3	142.4	146.0
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	78.3	93.0	67.4
Global Property | Client location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	121.0	100.0	86.0
Global Property | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	31.6	54.4	192.9
Global Property | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	304.9	239.4	238.3
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	270.6	225.2	50.1
Global Property | Underwriting location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	47.9	37.2	32.9
Global A&H | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	379.8	341.5	277.6
Global A&H | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	307.7	284.5	211.7
Global A&H | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	30.4	28.2	34.0
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	12.5	9.5	9.4
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	29.2	19.3	22.5
Global A&H | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	109.5	98.2	111.6
Global A&H | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	224.9	196.4	164.2
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	44.7	46.4	1.4
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	0.7	0.5	0.4
Specialty & Casualty | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	292.7	193.0	137.4
Specialty & Casualty | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	173.4	59.0	28.4
Specialty & Casualty | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	74.8	91.7	78.9
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	12.8	8.7	11.3
Specialty & Casualty | Client location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	31.7	33.6	18.8
Specialty & Casualty | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	9.8	(1.0)	6.5
Specialty & Casualty | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	162.3	147.5	128.8
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	118.5	43.9
Specialty & Casualty | Underwriting location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	2.1	2.6	2.1
Runoff & Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	29.6	(0.5)	8.9
Runoff & Other | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	29.2	(1.2)	8.1
Runoff & Other | 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		0.2	0.2
Runoff & Other | Client location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.4	0.5	0.6
Runoff & Other | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	29.2	(1.1)	8.3
Runoff & Other | Underwriting location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	$ 0.4	$ 0.6	$ 0.6