Segment information - Segment results (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) product segment	Jun. 30, 2019 USD ($)
Segment information
Number of reportable segments | segment			4
Gross written premiums	$ 322.6	$ 487.1	$ 1,140.2	$ 1,109.4
Net written premiums	306.0	401.7	935.9	886.5
Net earned insurance and reinsurance premiums	369.8	370.7	804.5	682.6
Loss and allocated LAE	(228.0)	(266.8)	(642.1)	(441.3)
Insurance and reinsurance acquisition expenses	(78.1)	(77.0)	(152.8)	(140.3)
Technical profit (loss)	63.7	26.9	9.6	101.0
Unallocated LAE	(12.3)	(11.2)	(25.3)	(20.6)
Other underwriting expenses	(36.3)	(35.5)	(74.3)	(70.8)
Underwriting income (loss)	15.1	(19.8)	(90.0)	9.6
Service fee revenue	11.4	16.6	36.3	41.9
General and administrative expenses, MGU plus Runoff & Other	(23.9)	(28.2)	(56.0)	(52.6)
Underwriting income (loss), including net service fee income	14.1	(19.2)	(81.8)	18.5
Net investment income	14.8	24.4	28.3	44.5
Net realized investment gains	7.1	15.6	27.4	24.6
Net unrealized investment gains	8.7	15.5	(35.1)	89.5
Net foreign exchange (losses)	(16.1)	(0.6)	2.4	4.5
Other revenue	(1.2)	(1.2)	(21.8)	(6.9)
General and administrative expenses	(11.5)	(12.2)	(27.9)	(19.6)
Intangible asset amortization expenses	(4.0)	(4.0)	(7.9)	(7.9)
Interest expense on debt	(7.9)	(8.0)	(15.7)	(15.6)
Pre-tax income (loss)	$ 4.0	$ 10.3	$ (132.1)	$ 131.6
Underwriting Ratios
Loss ratio	65.00%	75.00%	83.00%	67.70%
Acquisition expense ratio	21.10%	20.80%	19.00%	20.60%
Other underwriting expense ratio	9.80%	9.60%	9.20%	10.40%
Combined ratio	95.90%	105.40%	111.20%	98.70%
Goodwill and intangible assets	$ 572.7	$ 588.3	$ 572.7	$ 588.3
Managing general underwriters
Segment information
General and administrative expenses, MGU plus Runoff & Other	(12.4)	(16.0)	(28.1)	(33.0)
Corporate Eliminations
Segment information
Insurance and reinsurance acquisition expenses	10.2	12.6	20.6	22.9
Technical profit (loss)	10.2	12.6	20.6	22.9
Unallocated LAE	(4.7)	(4.2)	(10.1)	(7.6)
Other underwriting expenses	(4.7)	(4.8)	(9.7)	(7.5)
Underwriting income (loss)	0.8	3.6	0.8	7.8
Service fee revenue	(11.7)	(13.7)	(22.7)	(24.7)
Corporate Eliminations | Managing general underwriters
Segment information
Unallocated LAE	6.2	5.3	12.2	9.4
Other underwriting expenses	4.7	4.8	9.7	7.5
Global Reinsurance
Segment information
Net written premiums	220.5	266.7	565.9	602.6
Global Reinsurance | Reportable segments
Segment information
Gross written premiums	213.5	317.1	678.8	752.1
Net written premiums	220.5	266.7	565.9	602.6
Net earned insurance and reinsurance premiums	240.9	244.9	475.9	456.2
Loss and allocated LAE	(148.5)	(188.5)	(405.7)	(296.3)
Insurance and reinsurance acquisition expenses	(53.7)	(50.0)	(104.6)	(95.6)
Technical profit (loss)	38.7	6.4	(34.4)	64.3
Unallocated LAE	(5.2)	(4.7)	(10.2)	(8.7)
Other underwriting expenses	(20.5)	(21.6)	(41.8)	(43.2)
Underwriting income (loss)	13.0	(19.9)	(86.4)	12.4
Underwriting income (loss), including net service fee income	$ 13.0	$ (19.9)	$ (86.4)	$ 12.4
Underwriting Ratios
Loss ratio	63.80%	78.90%	87.40%	66.90%
Acquisition expense ratio	22.30%	20.40%	22.00%	21.00%
Other underwriting expense ratio	8.50%	8.80%	8.80%	9.50%
Combined ratio	94.60%	108.10%	118.20%	97.40%
Global A&H
Segment information
Net written premiums	$ 62.0	$ 120.6	$ 262.7	$ 255.5
Global A&H | Reportable segments
Segment information
Gross written premiums	83.7	152.8	345.8	322.1
Net written premiums	62.0	120.6	262.7	255.5
Net earned insurance and reinsurance premiums	113.2	118.8	231.1	214.9
Loss and allocated LAE	(69.6)	(71.8)	(149.9)	(135.0)
Insurance and reinsurance acquisition expenses	(31.5)	(36.0)	(62.3)	(62.6)
Technical profit (loss)	12.1	11.0	18.9	17.3
Unallocated LAE	(0.6)	(2.0)	(2.3)	(3.5)
Other underwriting expenses	(6.7)	(5.9)	(12.3)	(12.0)
Underwriting income (loss)	4.8	3.1	4.3	1.8
Service fee revenue	23.1	30.3	59.0	66.6
Underwriting income (loss), including net service fee income	$ 6.0	$ 8.3	$ 16.2	$ 20.3
Underwriting Ratios
Loss ratio	62.00%	62.10%	65.90%	64.40%
Acquisition expense ratio	27.80%	30.30%	27.00%	29.10%
Other underwriting expense ratio	5.90%	5.00%	5.30%	5.60%
Combined ratio	95.70%	97.40%	98.20%	99.10%
Goodwill and intangible assets	$ 564.6	$ 580.2	$ 564.6	$ 580.2
Global A&H | Reportable segments | Managing general underwriters
Segment information
Unallocated LAE	(6.2)	(5.3)	(12.2)	(9.4)
Other underwriting expenses	(4.7)	(4.8)	(9.7)	(7.5)
General and administrative expenses, MGU plus Runoff & Other	(11.0)	(15.0)	$ (25.2)	(31.2)
U.S. Specialty
Segment information
Number of core products | product			4
Net written premiums	23.8	14.1	$ 39.0	27.7
U.S. Specialty | Reportable segments
Segment information
Gross written premiums	25.5	16.0	46.3	32.6
Net written premiums	23.8	14.1	39.0	27.7
Net earned insurance and reinsurance premiums	15.2	6.7	28.0	10.8
Loss and allocated LAE	(10.4)	(4.1)	(18.1)	(6.5)
Insurance and reinsurance acquisition expenses	(3.4)	(1.8)	(6.4)	(2.5)
Technical profit (loss)	1.4	0.8	3.5	1.8
Unallocated LAE	(0.1)	(0.1)	(0.2)	(0.1)
Other underwriting expenses	(2.9)	(2.1)	(8.1)	(4.9)
Underwriting income (loss)	(1.6)	(1.4)	(4.8)	(3.2)
Underwriting income (loss), including net service fee income	$ (1.6)	$ (1.4)	$ (4.8)	$ (3.2)
Underwriting Ratios
Loss ratio	69.10%	62.70%	65.40%	61.10%
Acquisition expense ratio	22.40%	26.90%	22.90%	23.10%
Other underwriting expense ratio	19.10%	31.30%	28.90%	45.40%
Combined ratio	110.60%	120.90%	117.20%	129.60%
U.S. Specialty | Reportable segments | Minimum
Segment information
Pre-agreed domestic and export sales of goods and services coverage period			60 days
U.S. Specialty | Reportable segments | Maximum
Segment information
Pre-agreed domestic and export sales of goods and services coverage period			120 days
Runoff & Other
Segment information
Net written premiums	$ (0.3)	$ 0.3	$ 68.3	$ 0.7
Runoff & Other | Reportable segments
Segment information
Gross written premiums	(0.1)	1.2	69.3	2.6
Net written premiums	(0.3)	0.3	68.3	0.7
Net earned insurance and reinsurance premiums	0.5	0.3	69.5	0.7
Loss and allocated LAE	0.5	(2.4)	(68.4)	(3.5)
Insurance and reinsurance acquisition expenses	0.3	(1.8)	(0.1)	(2.5)
Technical profit (loss)	1.3	(3.9)	1.0	(5.3)
Unallocated LAE	(1.7)	(0.2)	(2.5)	(0.7)
Other underwriting expenses	(1.5)	(1.1)	(2.4)	(3.2)
Underwriting income (loss)	(1.9)	(5.2)	(3.9)	(9.2)
Underwriting income (loss), including net service fee income	(3.3)	(6.2)	(6.8)	(11.0)
Underwriting Ratios
Goodwill and intangible assets	8.1	8.1	8.1	8.1
Runoff & Other | Reportable segments | Managing general underwriters
Segment information
General and administrative expenses, MGU plus Runoff & Other	$ (1.4)	$ (1.0)	$ (2.9)	$ (1.8)