Segment information - Net premiums written by client location and underwriting location by reportable segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	$ 629.9	$ 484.8
Client location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	368.3	241.5
Client location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	180.3	157.6
Client location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	35.9	34.0
Client location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	45.4	51.7
Underwriting location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	287.0	160.5
Underwriting location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	278.1	270.7
Underwriting location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	48.1	35.9
Underwriting location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	16.7	17.7
Global Reinsurance | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	345.4	335.9
Global Reinsurance | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	104.3	116.6
Global Reinsurance | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	172.8	149.5
Global Reinsurance | 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	29.4	29.0
Global Reinsurance | Client location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	38.9	40.8
Global Reinsurance | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	72.6	75.7
Global Reinsurance | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	208.2	207.0
Global Reinsurance | 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	48.1	35.9
Global Reinsurance | Underwriting location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	16.5	17.3
Global A&H | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	200.7	134.9
Global A&H | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	180.7	111.1
Global A&H | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	7.2	8.0
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	6.5	5.0
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	6.3	10.8
Global A&H | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	131.2	71.0
Global A&H | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	69.5	63.6
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	0.0	0.0
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	0.0	0.3
U.S. Specialty | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	15.2	13.6
U.S. Specialty | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	15.2	13.6
U.S. Specialty | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0
U.S. Specialty | 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	0.0	0.0
U.S. Specialty | Client location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0
U.S. Specialty | Underwriting location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	15.2	13.6
U.S. Specialty | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0
U.S. Specialty | 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	0.0	0.0
U.S. Specialty | Underwriting location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0
Runoff & Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	68.6	0.4
Runoff & Other | Client location: United States | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	68.1	0.2
Runoff & Other | Client location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.3	0.1
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	0.0	0.0
Runoff & Other | Client location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.2	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	68.0	0.2
Runoff & Other | Underwriting location: Europe | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.4	0.1
Runoff & Other | 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	0.0	0.0
Runoff & Other | Underwriting location: Asia and Other | Reportable segments
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	$ 0.2	$ 0.1