Segment information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) segment product	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment information
Number of reportable segments | segment	4
Gross written premiums	$ 1,821.0	$ 1,439.3	$ 1,269.0
Net written premiums	1,357.1	1,090.2	938.1
Net earned insurance and reinsurance premiums	1,262.3	1,035.3	890.1
Loss and allocated LAE	(863.1)	(785.1)	(495.2)
Insurance and reinsurance acquisition expenses	(255.4)	(197.2)	(210.3)
Technical profit (loss)	143.8	53.0	184.6
Unallocated LAE	(36.9)	(26.1)	(24.1)
Other underwriting expenses	(146.2)	(106.1)	(107.3)
Underwriting (loss) income	(39.3)	(79.2)	53.2
Service fee revenue	71.0	23.0	4.3
General and administrative expenses	77.9	91.9	85.1
Underwriting (loss) income, including net service fee income	(25.0)	(105.0)	51.3
Net investment income	71.4	56.8	56.2
Net realized investment gains (losses)	2.3	(27.2)	288.3
Net unrealized investment gains (losses)	(23.2)	(10.5)	(238.2)
Net foreign exchange gains (losses)	22.7	9.2	(11.0)
Gain on revaluation of contingent consideration	9.6	48.8
Other revenue	41.7	(1.3)	4.8
General and administrative expenses	(21.2)	(43.1)	(78.9)
Intangible asset amortization expenses	(15.8)	(10.2)
Impairment of intangible assets	(8.0)	(5.0)
Interest expense on debt	(30.8)	(22.4)	(34.6)
Pre-tax income (loss)	$ 23.7	$ (109.9)	$ 37.9
Underwriting Ratios
Loss ratio	71.30%	78.40%	58.30%
Acquisition expense ratio	20.20%	19.00%	23.60%
Other underwriting expense ratio	11.60%	10.20%	12.10%
Combined ratio	103.10%	107.60%	94.00%
Goodwill and intangible assets	$ 596.2	$ 617.3	$ 5.0
Managing general underwriters
Segment information
General and administrative expenses	56.7	48.8	6.2
Corporate Elimination
Segment information
Insurance and reinsurance acquisition expenses	43.7	42.9
Technical profit (loss)	43.7	42.9
Unallocated LAE	(13.0)
Other underwriting expenses	(16.1)
Underwriting (loss) income	14.6	42.9
Service fee revenue	(44.7)	(42.9)
Corporate Elimination | Managing general underwriters
Segment information
Unallocated LAE	14.0
Other underwriting expenses	16.1
Global Property | Reportable segments
Segment information
Gross written premiums	962.4	732.1	634.9
Net written premiums	655.0	556.2	514.2
Net earned insurance and reinsurance premiums	636.0	564.4	481.8
Loss and allocated LAE	(518.2)	(499.5)	(256.6)
Insurance and reinsurance acquisition expenses	(121.1)	(112.9)	(100.3)
Technical profit (loss)	(3.3)	(48.0)	124.9
Unallocated LAE	(10.5)	(12.9)	(11.9)
Other underwriting expenses	(68.2)	(63.3)	(65.1)
Underwriting (loss) income	(82.0)	(124.2)	47.9
Underwriting (loss) income, including net service fee income	$ (82.0)	$ (124.2)	$ 47.9
Underwriting Ratios
Loss ratio	83.10%	90.80%	55.70%
Acquisition expense ratio	19.00%	20.00%	20.80%
Other underwriting expense ratio	10.70%	11.20%	13.50%
Combined ratio	112.80%	122.00%	90.00%
Global A&H | Reportable segments
Segment information
Gross written premiums	$ 500.6	$ 494.6	$ 436.1
Net written premiums	379.8	341.5	277.6
Net earned insurance and reinsurance premiums	357.6	306.8	272.2
Loss and allocated LAE	(189.0)	(175.0)	(164.8)
Insurance and reinsurance acquisition expenses	(109.7)	(89.6)	(71.7)
Technical profit (loss)	58.9	42.2	35.7
Unallocated LAE	(5.9)	(4.8)	(6.8)
Other underwriting expenses	(27.4)	(23.4)	(23.3)
Underwriting (loss) income	25.6	14.0	5.6
Service fee revenue	115.7	65.9
Underwriting (loss) income, including net service fee income	$ 57.6	$ 35.1	$ 5.6
Underwriting Ratios
Loss ratio	54.50%	58.60%	63.00%
Acquisition expense ratio	30.70%	29.20%	26.30%
Other underwriting expense ratio	7.70%	7.60%	8.60%
Combined ratio	92.90%	95.40%	97.90%
Goodwill and intangible assets	$ 588.1	$ 612.3
Global A&H | Reportable segments | Managing general underwriters
Segment information
Unallocated LAE	(14.0)
Other underwriting expenses	(16.1)
General and administrative expenses	$ 53.6	44.8
Specialty & Casualty | Reportable segments
Segment information
Number of core products | product	4
Gross written premiums	$ 325.1	218.1	$ 170.7
Net written premiums	292.7	193.0	137.4
Net earned insurance and reinsurance premiums	239.5	163.2	135.2
Loss and allocated LAE	(142.9)	(99.6)	(71.2)
Insurance and reinsurance acquisition expenses	(65.5)	(41.1)	(37.7)
Technical profit (loss)	31.1	22.5	26.3
Unallocated LAE	(5.9)	(5.3)	(4.4)
Other underwriting expenses	(28.2)	(16.5)	(15.1)
Underwriting (loss) income	(3.0)	0.7	6.8
Underwriting (loss) income, including net service fee income	$ (3.0)	$ 0.7	$ 6.8
Underwriting Ratios
Loss ratio	62.10%	64.30%	55.90%
Acquisition expense ratio	27.30%	25.20%	27.90%
Other underwriting expense ratio	11.80%	10.10%	11.20%
Combined ratio	101.20%	99.60%	95.00%
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	$ 32.9	$ (5.5)	$ 27.3
Net written premiums	29.6	(0.5)	8.9
Net earned insurance and reinsurance premiums	29.2	0.9	0.9
Loss and allocated LAE	(13.0)	(11.0)	(2.6)
Insurance and reinsurance acquisition expenses	(2.8)	3.5	(0.6)
Technical profit (loss)	13.4	(6.6)	(2.3)
Unallocated LAE	(1.6)	(3.1)	(1.0)
Other underwriting expenses	(6.3)	(2.9)	(3.8)
Underwriting (loss) income	5.5	(12.6)	(7.1)
Service fee revenue			4.3
Underwriting (loss) income, including net service fee income	2.4	(16.6)	(9.0)
Underwriting Ratios
Goodwill and intangible assets	8.1	5.0	5.0
Runoff & Other | Reportable segments | Managing general underwriters
Segment information
General and administrative expenses	$ 3.1	$ 4.0	$ 6.2