Segment information - Segment results (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) product segment	Sep. 30, 2019 USD ($)
Segment information
Number of reportable segments | segment			4
Gross written premiums	$ 356.0	$ 413.7	$ 1,496.2	$ 1,523.1
Net written premiums	282.3	322.3	1,218.2	1,208.8
Net earned insurance and reinsurance premiums	372.6	374.2	1,177.1	1,056.8
Loss and allocated LAE	(291.3)	(334.4)	(925.1)	(775.7)
Insurance and reinsurance acquisition expenses	(83.6)	(75.1)	(236.4)	(215.4)
Technical (loss) profit	(2.3)	(35.3)	15.6	65.7
Unallocated LAE	(14.1)	(14.2)	(47.7)	(34.8)
Other underwriting expenses	(41.3)	(35.4)	(115.6)	(106.2)
Underwriting (loss) income	(57.7)	(84.9)	(147.7)	(75.3)
Service fee revenue	16.5	16.4	52.8	58.3
General and administrative expenses, MGU plus Runoff & Other	(34.4)	(28.0)	(90.4)	(80.6)
Underwriting (loss) income, including net service fee income	(53.1)	(84.8)	(134.9)	(66.3)
Net investment income	1.7	22.8	30.0	67.3
Net realized investment gains	(3.8)	15.3	23.6	39.9
Net unrealized investment gains	31.2	53.9	(3.9)	143.4
Net foreign exchange (losses)	(21.3)	4.9	(18.9)	9.4
Other revenue	(0.7)	(0.1)	(22.5)	(7.0)
General and administrative expenses	(22.5)	(11.7)	(50.4)	(31.3)
Intangible asset amortization expenses	(3.9)	(3.9)	(11.8)	(11.8)
Interest expense on debt	(8.1)	(7.7)	(23.8)	(23.3)
Pre-tax income (loss)	$ (80.5)	$ (11.3)	$ (212.6)	$ 120.3
Underwriting Ratios
Loss ratio	82.00%	93.20%	82.60%	76.70%
Acquisition expense ratio	22.40%	20.10%	20.10%	20.40%
Other underwriting expense ratio	11.10%	9.50%	9.80%	10.00%
Combined ratio	115.50%	122.80%	112.50%	107.10%
Goodwill and intangible assets	$ 568.8	$ 584.2	$ 568.8	$ 584.2
Managing general underwriters
Segment information
General and administrative expenses, MGU plus Runoff & Other	(11.9)	(16.3)	(40.0)	(49.3)
Corporate Eliminations
Segment information
Insurance and reinsurance acquisition expenses	9.6	13.8	30.2	36.7
Technical (loss) profit	9.6	13.8	30.2	36.7
Unallocated LAE	(4.7)	(3.5)	(14.8)	(11.1)
Other underwriting expenses	(4.7)	(3.7)	(14.4)	(11.2)
Underwriting (loss) income	0.2	6.6	1.0	14.4
Service fee revenue	(10.7)	(14.6)	(33.4)	(39.3)
Corporate Eliminations | Managing general underwriters
Segment information
Unallocated LAE	5.8	4.3	18.0	13.7
Other underwriting expenses	4.7	3.7	14.4	11.2
Global Reinsurance
Segment information
Net written premiums	192.2	202.6	758.1	805.2
Global Reinsurance | Reportable segments
Segment information
Gross written premiums	248.2	256.3	927.0	1,008.4
Net written premiums	192.2	202.6	758.1	805.2
Net earned insurance and reinsurance premiums	243.5	249.5	719.4	705.7
Loss and allocated LAE	(194.5)	(261.7)	(600.2)	(558.0)
Insurance and reinsurance acquisition expenses	(51.8)	(54.3)	(156.4)	(149.9)
Technical (loss) profit	(2.8)	(66.5)	(37.2)	(2.2)
Unallocated LAE	(7.7)	(8.4)	(17.9)	(17.1)
Other underwriting expenses	(25.3)	(20.8)	(67.1)	(64.0)
Underwriting (loss) income	(35.8)	(95.7)	(122.2)	(83.3)
Underwriting (loss) income, including net service fee income	$ (35.8)	$ (95.7)	$ (122.2)	$ (83.3)
Underwriting Ratios
Loss ratio	83.00%	108.30%	85.90%	81.50%
Acquisition expense ratio	21.30%	21.80%	21.70%	21.20%
Other underwriting expense ratio	10.40%	8.30%	9.30%	9.10%
Combined ratio	114.70%	138.40%	116.90%	111.80%
Global A&H
Segment information
Net written premiums	$ 72.2	$ 104.6	$ 334.9	$ 360.1
Global A&H | Reportable segments
Segment information
Gross written premiums	88.8	137.4	434.6	459.5
Net written premiums	72.2	104.6	334.9	360.1
Net earned insurance and reinsurance premiums	114.3	115.1	345.4	330.0
Loss and allocated LAE	(84.6)	(63.6)	(234.5)	(198.6)
Insurance and reinsurance acquisition expenses	(36.6)	(32.5)	(98.9)	(95.1)
Technical (loss) profit	(6.9)	19.0	12.0	36.3
Unallocated LAE	(0.5)	(2.0)	(2.8)	(5.5)
Other underwriting expenses	(5.7)	(6.8)	(18.0)	(18.8)
Underwriting (loss) income	(13.1)	10.2	(8.8)	12.0
Service fee revenue	26.3	31.0	85.3	97.6
Underwriting (loss) income, including net service fee income	$ (8.3)	$ 18.1	$ 7.9	$ 38.4
Underwriting Ratios
Loss ratio	74.50%	57.00%	68.70%	61.80%
Acquisition expense ratio	32.00%	28.20%	28.60%	28.80%
Other underwriting expense ratio	5.00%	5.90%	5.20%	5.70%
Combined ratio	111.50%	91.10%	102.50%	96.30%
Goodwill and intangible assets	$ 560.7	$ 576.1	$ 560.7	$ 576.1
Global A&H | Reportable segments | Managing general underwriters
Segment information
Unallocated LAE	(5.8)	(4.3)	(18.0)	(13.7)
Other underwriting expenses	(4.7)	(3.7)	(14.4)	(11.2)
General and administrative expenses, MGU plus Runoff & Other	(11.0)	(15.1)	$ (36.2)	(46.3)
U.S. Specialty
Segment information
Number of core products | product			4
Net written premiums	20.5	14.3	$ 59.5	42.0
U.S. Specialty | Reportable segments
Segment information
Gross written premiums	22.6	17.9	68.9	50.5
Net written premiums	20.5	14.3	59.5	42.0
Net earned insurance and reinsurance premiums	17.2	9.1	45.2	19.9
Loss and allocated LAE	(11.1)	(8.2)	(29.2)	(14.7)
Insurance and reinsurance acquisition expenses	(4.0)	(2.0)	(10.4)	(4.5)
Technical (loss) profit	2.1	(1.1)	5.6	0.7
Unallocated LAE	(0.2)	(0.1)	(0.4)	(0.2)
Other underwriting expenses	(4.3)	(2.7)	(12.4)	(7.6)
Underwriting (loss) income	(2.4)	(3.9)	(7.2)	(7.1)
Underwriting (loss) income, including net service fee income	$ (2.4)	$ (3.9)	$ (7.2)	$ (7.1)
Underwriting Ratios
Loss ratio	65.70%	91.20%	65.50%	74.90%
Acquisition expense ratio	23.30%	22.00%	23.00%	22.60%
Other underwriting expense ratio	25.00%	29.70%	27.40%	38.20%
Combined ratio	114.00%	142.90%	115.90%	135.70%
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	$ (2.6)	$ 0.8	$ 65.7	$ 1.5
Runoff & Other | Reportable segments
Segment information
Gross written premiums	(3.6)	2.1	65.7	4.7
Net written premiums	(2.6)	0.8	65.7	1.5
Net earned insurance and reinsurance premiums	(2.4)	0.5	67.1	1.2
Loss and allocated LAE	(1.1)	(0.9)	(61.2)	(4.4)
Insurance and reinsurance acquisition expenses	(0.8)	(0.1)	(0.9)	(2.6)
Technical (loss) profit	(4.3)	(0.5)	5.0	(5.8)
Unallocated LAE	(1.0)	(0.2)	(11.8)	(0.9)
Other underwriting expenses	(1.3)	(1.4)	(3.7)	(4.6)
Underwriting (loss) income	(6.6)	(2.1)	(10.5)	(11.3)
Service fee revenue	0.9		0.9
Underwriting (loss) income, including net service fee income	(6.6)	(3.3)	(13.4)	(14.3)
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	$ (0.9)	$ (1.2)	$ (3.8)	$ (3.0)