Segment information - Net premiums written by client location and underwriting location by reportable segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	$ 306.0	$ 401.7	$ 935.9	$ 886.5
Client location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	180.9	269.5	549.2	511.0
Client location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	48.0	33.6	228.3	191.2
Client location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	20.2	18.7	56.1	52.7
Client location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	56.9	79.9	102.3	131.6
Underwriting location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	132.0	181.3	419.0	341.8
Underwriting location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	119.7	161.0	397.8	431.7
Underwriting location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	36.6	40.7	84.7	76.6
Underwriting location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	17.7	18.7	34.4	36.4
Global Reinsurance
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	220.5	266.7	565.9	602.6
Global Reinsurance | Client location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	106.9	152.7	211.2	269.3
Global Reinsurance | Client location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	42.7	28.5	215.5	178.0
Global Reinsurance | Client location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	18.9	16.7	48.3	45.7
Global Reinsurance | Client location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	52.0	68.8	90.9	109.6
Global Reinsurance | Underwriting location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	83.2	104.9	155.8	180.6
Global Reinsurance | Underwriting location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	83.2	102.6	291.4	309.6
Global Reinsurance | Underwriting location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	36.6	40.7	84.7	76.6
Global Reinsurance | Underwriting location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	17.5	18.5	34.0	35.8
Global A&H
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	62.0	120.6	262.7	255.5
Global A&H | Client location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	50.7	102.5	231.4	213.6
Global A&H | Client location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	5.2	5.1	12.4	13.1
Global A&H | Client location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	1.3	2.0	7.8	7.0
Global A&H | Client location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	4.8	11.0	11.1	21.8
Global A&H | Underwriting location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	25.4	62.1	156.6	133.1
Global A&H | Underwriting location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	36.5	58.4	106.0	122.0
Global A&H | Underwriting location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0	0.0	0.0
Global A&H | Underwriting location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.1	0.1	0.1	0.4
U.S. Specialty
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	23.8	14.1	39.0	27.7
U.S. Specialty | Client location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	23.8	14.1	39.0	27.7
U.S. Specialty | Client location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0	0.0	0.0
U.S. Specialty | Client location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0	0.0	0.0
U.S. Specialty | Client location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0	0.0	0.0
U.S. Specialty | Underwriting location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	23.8	14.1	39.0	27.7
U.S. Specialty | Underwriting location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0	0.0	0.0
U.S. Specialty | Underwriting location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0	0.0	0.0
U.S. Specialty | Underwriting location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0	0.0	0.0
Runoff & Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	(0.3)	0.3	68.3	0.7
Runoff & Other | Client location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	(0.5)	0.2	67.6	0.4
Runoff & Other | Client location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.1	0.0	0.4	0.1
Runoff & Other | Client location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0	0.0	0.0
Runoff & Other | Client location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.1	0.1	0.3	0.2
Runoff & Other | Underwriting location: United States
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	(0.4)	0.2	67.6	0.4
Runoff & Other | Underwriting location: Europe
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0	0.4	0.1
Runoff & Other | Underwriting location: Canada, the Caribbean, Bermuda and Latin America
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	0.0	0.0	0.0	0.0
Runoff & Other | Underwriting location: Asia and Other
Net premiums written by client location and underwriting location by reportable segment
Net written premiums	$ 0.1	$ 0.1	$ 0.3	$ 0.2