Segment information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) segment product	Jun. 30, 2018 USD ($)
Segment information
Number of reportable segments | segment			4
Gross written premiums	$ 487.1	$ 505.0	$ 1,109.4	$ 1,120.2
Net written premiums	401.7	320.3	886.5	789.7
Net earned insurance and reinsurance premiums	370.7	308.9	682.6	593.4
Loss and allocated LAE	(266.8)	(141.1)	(441.3)	(276.5)
Insurance and reinsurance acquisition expenses	(77.0)	(66.8)	(140.3)	(129.8)
Technical profit	26.9	101.0	101.0	187.1
Unallocated LAE	(11.2)	(10.3)	(20.6)	(15.9)
Other underwriting expenses	(35.5)	(38.2)	(70.8)	(81.4)
Underwriting income (loss)	(19.8)	52.5	9.6	89.8
Service fee revenue	16.6	17.6	41.9	40.1
General and administrative expenses	(28.2)	(24.2)	(52.6)	(38.5)
Underwriting income (loss), including net service fee income	(19.2)	54.9	18.5	104.1
Net investment income	24.4	19.2	44.5	30.0
Net realized investment (losses) gains	15.6	7.8	24.6	4.1
Net unrealized investment (losses) gains	15.5	24.7	89.5	40.7
Net foreign exchange gains (losses)	(0.6)	25.6	4.5	22.1
Other revenue	(1.2)	38.0	(6.9)	38.9
General and administrative expenses	(12.2)	(9.0)	(19.6)	(12.7)
Intangible asset amortization expenses	(4.0)	(4.0)	(7.9)	(7.9)
Interest expense on debt	(8.0)	(7.8)	(15.6)	(15.5)
Pre-tax income	$ 10.3	$ 149.4	$ 131.6	$ 203.8
Underwriting Ratios
Loss ratio	75.00%	49.00%	67.70%	49.30%
Acquisition expense ratio	20.80%	21.60%	20.60%	21.90%
Other underwriting expense ratio	9.60%	12.40%	10.40%	13.70%
Combined ratio	105.40%	83.00%	98.70%	84.90%
Goodwill and intangible assets	$ 588.3	$ 609.2	$ 588.3	$ 609.2
Managing general underwriters
Segment information
General and administrative expenses	(16.0)	(15.2)	(33.0)	(25.8)
Corporate Elimination
Segment information
Insurance and reinsurance acquisition expenses	12.6	9.8	22.9	20.1
Technical profit	12.6	9.8	22.9	20.1
Unallocated LAE	(4.2)	(5.1)	(7.6)	(5.1)
Other underwriting expenses	(4.8)	(3.5)	(7.5)	(11.9)
Underwriting income (loss)	3.6	1.2	7.8	3.1
Service fee revenue	(13.7)	(9.8)	(24.7)	(20.1)
Corporate Elimination | Managing general underwriters
Segment information
Unallocated LAE	5.3	5.1	9.4	5.1
Other underwriting expenses	4.8	3.5	7.5	11.9
Global Property | Reportable segments
Segment information
Gross written premiums	236.2	325.4	566.9	672.0
Net written premiums	191.6	177.0	432.9	424.2
Net earned insurance and reinsurance premiums	164.3	167.5	304.0	303.6
Loss and allocated LAE	(131.3)	(68.2)	(193.9)	(138.6)
Insurance and reinsurance acquisition expenses	(27.2)	(34.1)	(53.0)	(63.4)
Technical profit	5.8	65.2	57.1	101.6
Unallocated LAE	(2.6)	(2.5)	(4.7)	(4.4)
Other underwriting expenses	(17.0)	(18.1)	(33.2)	(35.5)
Underwriting income (loss)	(13.8)	44.6	19.2	61.7
Underwriting income (loss), including net service fee income	$ (13.8)	$ 44.6	$ 19.2	$ 61.7
Underwriting Ratios
Loss ratio	81.50%	42.20%	65.30%	47.10%
Acquisition expense ratio	16.60%	20.40%	17.40%	20.90%
Other underwriting expense ratio	10.30%	10.80%	10.90%	11.70%
Combined ratio	108.40%	73.40%	93.60%	79.70%
Global A&H | Reportable segments
Segment information
Gross written premiums	$ 152.8	$ 112.3	$ 322.1	$ 257.9
Net written premiums	120.6	82.8	255.5	198.3
Net earned insurance and reinsurance premiums	118.8	80.8	214.9	168.8
Loss and allocated LAE	(71.8)	(40.9)	(135.0)	(86.7)
Insurance and reinsurance acquisition expenses	(36.0)	(26.4)	(62.6)	(55.6)
Technical profit	11.0	13.5	17.3	26.5
Unallocated LAE	(2.0)	(1.0)	(3.5)	(2.6)
Other underwriting expenses	(5.9)	(6.3)	(12.0)	(14.3)
Underwriting income (loss)	3.1	6.2	1.8	9.6
Service fee revenue	30.3	27.4	66.6	60.2
Underwriting income (loss), including net service fee income	$ 8.3	$ 10.8	$ 20.3	$ 29.1
Underwriting Ratios
Loss ratio	62.10%	51.90%	64.40%	52.90%
Acquisition expense ratio	30.30%	32.70%	29.10%	32.90%
Other underwriting expense ratio	5.00%	7.80%	5.60%	8.50%
Combined ratio	97.40%	92.40%	99.10%	94.30%
Goodwill and intangible assets	$ 580.2	$ 604.2	$ 580.2	$ 604.2
Global A&H | Reportable segments | Managing general underwriters
Segment information
Unallocated LAE	(5.3)	(5.1)	(9.4)	(5.1)
Other underwriting expenses	(4.8)	(3.5)	(7.5)	(11.9)
General and administrative expenses	(15.0)	(14.2)	$ (31.2)	(23.7)
Specialty & Casualty | Reportable segments
Segment information
Number of core products | product			4
Gross written premiums	96.9	60.9	$ 217.8	176.4
Net written premiums	89.2	55.1	197.4	155.7
Net earned insurance and reinsurance premiums	87.3	55.5	163.0	109.9
Loss and allocated LAE	(61.3)	(31.7)	(108.9)	(53.3)
Insurance and reinsurance acquisition expenses	(24.6)	(14.6)	(45.1)	(28.7)
Technical profit	1.4	9.2	9.0	27.9
Unallocated LAE	(2.2)	(1.7)	(4.1)	(2.9)
Other underwriting expenses	(6.7)	(7.9)	(14.9)	(15.9)
Underwriting income (loss)	(7.5)	(0.4)	(10.0)	9.1
Underwriting income (loss), including net service fee income	$ (7.5)	$ (0.4)	$ (10.0)	$ 9.1
Underwriting Ratios
Loss ratio	72.70%	60.20%	69.30%	51.10%
Acquisition expense ratio	28.20%	26.30%	27.70%	26.10%
Other underwriting expense ratio	7.70%	14.20%	9.10%	14.50%
Combined ratio	108.60%	100.70%	106.10%	91.70%
Specialty & Casualty | Reportable segments | Minimum
Segment information
Pre-agreed domestic and export sales of goods and services coverage period			60 days
Specialty & Casualty | 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	$ 1.2	$ 6.4	$ 2.6	$ 13.9
Net written premiums	0.3	5.4	0.7	11.5
Net earned insurance and reinsurance premiums	0.3	5.1	0.7	11.1
Loss and allocated LAE	(2.4)	(0.3)	(3.5)	2.1
Insurance and reinsurance acquisition expenses	(1.8)	(1.5)	(2.5)	(2.2)
Technical profit	(3.9)	3.3	(5.3)	11.0
Unallocated LAE	(0.2)		(0.7)	(0.9)
Other underwriting expenses	(1.1)	(2.4)	(3.2)	(3.8)
Underwriting income (loss)	(5.2)	0.9	(9.2)	6.3
Underwriting income (loss), including net service fee income	(6.2)	(0.1)	(11.0)	4.2
Underwriting Ratios
Goodwill and intangible assets	8.1	5.0	8.1	5.0
Runoff & Other | Reportable segments | Managing general underwriters
Segment information
General and administrative expenses	$ (1.0)	$ (1.0)	$ (1.8)	$ (2.1)