Segment information - Segment results (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) product segment	Mar. 31, 2019 USD ($)
Segment information
Number of reportable segments | segment	4
Gross written premiums	$ 817.6	$ 622.3
Net written premiums	629.9	484.8
Net earned insurance and reinsurance premiums	434.7	311.9
Loss and allocated LAE	(414.1)	(174.5)
Insurance and reinsurance acquisition expenses	(74.7)	(63.3)
Technical profit	(54.1)	74.1
Unallocated LAE	(13.0)	(9.4)
Other underwriting expenses	(38.0)	(35.3)
Underwriting income (loss)	(105.1)	29.4
Service fee revenue	24.9	25.3
General and administrative expenses, MGU plus Runoff & Other	(32.1)	(24.4)
Underwriting (loss) income, including net service fee income	(95.9)	37.7
Net investment income	13.5	20.1
Net realized investment (losses) gains	20.3	9.0
Net unrealized investment (losses) gains	(43.8)	74.0
Net foreign exchange gains (losses)	18.5	5.1
Other revenue	(20.6)	(5.7)
General and administrative expenses	(16.4)	(7.4)
Intangible asset amortization expenses	(3.9)	(3.9)
Interest expense on debt	(7.8)	(7.6)
Pre-tax (loss) income	$ (136.1)	$ 121.3
Underwriting Ratios
Loss ratio	98.30%	59.00%
Acquisition expense ratio	17.20%	20.30%
Other underwriting expense ratio	8.70%	11.30%
Combined ratio	124.20%	90.60%
Goodwill and intangible assets	$ 576.7	$ 592.3
Managing general underwriters
Segment information
General and administrative expenses, MGU plus Runoff & Other	(15.7)	(17.0)
Corporate Elimination
Segment information
Insurance and reinsurance acquisition expenses	10.4	10.3
Technical profit	10.4	10.3
Unallocated LAE	(5.4)	(3.4)
Other underwriting expenses	(5.0)	(2.7)
Underwriting income (loss)	0.0	4.2
Service fee revenue	(11.0)	(11.0)
Corporate Elimination | Managing general underwriters
Segment information
Unallocated LAE	6.0	4.1
Other underwriting expenses	5.0	2.7
Global Reinsurance | Reportable segments
Segment information
Gross written premiums	465.3	435.0
Net written premiums	345.4	335.9
Net earned insurance and reinsurance premiums	235.0	211.3
Loss and allocated LAE	(257.2)	(107.8)
Insurance and reinsurance acquisition expenses	(50.9)	(45.6)
Technical profit	(73.1)	57.9
Unallocated LAE	(5.0)	(4.0)
Other underwriting expenses	(21.3)	(21.6)
Underwriting income (loss)	(99.4)	32.3
Underwriting (loss) income, including net service fee income	$ (99.4)	$ 32.3
Underwriting Ratios
Loss ratio	111.60%	52.90%
Acquisition expense ratio	21.70%	21.60%
Other underwriting expense ratio	9.10%	10.20%
Combined ratio	142.40%	84.70%
Global A&H | Reportable segments
Segment information
Gross written premiums	$ 262.1	$ 169.3
Net written premiums	200.7	134.9
Net earned insurance and reinsurance premiums	117.9	96.1
Loss and allocated LAE	(80.3)	(63.2)
Insurance and reinsurance acquisition expenses	(30.8)	(26.6)
Technical profit	6.8	6.3
Unallocated LAE	(1.7)	(1.5)
Other underwriting expenses	(5.6)	(6.1)
Underwriting income (loss)	(0.5)	(1.3)
Service fee revenue	35.9	36.3
Underwriting (loss) income, including net service fee income	$ 10.2	$ 12.0
Underwriting Ratios
Loss ratio	69.60%	67.30%
Acquisition expense ratio	26.10%	27.70%
Other underwriting expense ratio	4.70%	6.30%
Combined ratio	100.40%	101.30%
Goodwill and intangible assets	$ 568.6	$ 584.2
Global A&H | Reportable segments | Managing general underwriters
Segment information
Unallocated LAE	(6.0)	(4.1)
Other underwriting expenses	(5.0)	(2.7)
General and administrative expenses, MGU plus Runoff & Other	$ (14.2)	(16.2)
U.S. Specialty
Underwriting Ratios
Number of Core Products | product	4
U.S. Specialty | Reportable segments
Segment information
Gross written premiums	$ 20.8	16.6
Net written premiums	15.2	13.6
Net earned insurance and reinsurance premiums	12.8	4.1
Loss and allocated LAE	(7.7)	(2.4)
Insurance and reinsurance acquisition expenses	(3.0)	(0.7)
Technical profit	2.1	1.0
Unallocated LAE	(0.1)	0.0
Other underwriting expenses	(5.2)	(2.8)
Underwriting income (loss)	(3.2)	(1.8)
Underwriting (loss) income, including net service fee income	$ (3.2)	$ (1.8)
Underwriting Ratios
Loss ratio	60.90%	58.50%
Acquisition expense ratio	23.40%	17.10%
Other underwriting expense ratio	40.60%	68.30%
Combined ratio	124.90%	143.90%
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 | Reportable segments
Segment information
Gross written premiums	$ 69.4	$ 1.4
Net written premiums	68.6	0.4
Net earned insurance and reinsurance premiums	69.0	0.4
Loss and allocated LAE	(68.9)	(1.1)
Insurance and reinsurance acquisition expenses	(0.4)	(0.7)
Technical profit	(0.3)	(1.4)
Unallocated LAE	(0.8)	(0.5)
Other underwriting expenses	(0.9)	(2.1)
Underwriting income (loss)	(2.0)	(4.0)
Underwriting (loss) income, including net service fee income	(3.5)	(4.8)
Underwriting Ratios
Goodwill and intangible assets	8.1	8.1
Runoff & Other | Reportable segments | Managing general underwriters
Segment information
General and administrative expenses, MGU plus Runoff & Other	$ (1.5)	$ (0.8)