Segment information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) segment product	Mar. 31, 2018 USD ($)
Segment information
Number of reportable segments | segment	4
Gross written premiums	$ 622.3	$ 615.2
Net written premiums	484.8	469.4
Net earned insurance and reinsurance premiums	311.9	284.5
Loss and allocated LAE	(174.5)	(135.4)
Insurance and reinsurance acquisition expenses	(63.3)	(63.0)
Technical profit (loss)	74.1	86.1
Unallocated LAE	(9.4)	(5.6)
Other underwriting expenses	(35.3)	(43.2)
Underwriting (loss) income	29.4	37.3
Service fee revenue	25.3	22.5
General and administrative expenses	24.4	14.3
Underwriting (loss) income, including net service fee income	37.7	49.2
Net investment income	20.1	10.8
Net realized investment gains (losses)	9.0	(3.7)
Net unrealized investment gains	74.0	16.0
Net foreign exchange gains (losses)	5.1	(3.5)
Other revenue	(5.7)	0.9
General and administrative expenses	(7.4)	(3.7)
Intangible asset amortization expenses	(3.9)	(3.9)
Interest expense on debt	(7.6)	(7.7)
Pre-tax income	$ 121.3	$ 54.4
Underwriting Ratios
Loss ratio	59.00%	49.60%
Acquisition expense ratio	20.30%	22.10%
Other underwriting expense ratio	11.30%	15.20%
Combined ratio	90.60%	86.90%
Goodwill and intangible assets	$ 592.3	$ 613.6
Managing general underwriters
Segment information
General and administrative expenses	17.0	10.6
Corporate Elimination
Segment information
Insurance and reinsurance acquisition expenses	10.3	10.3
Technical profit (loss)	10.3	10.3
Unallocated LAE	(3.4)
Other underwriting expenses	(2.7)	(8.4)
Underwriting (loss) income	4.2	1.9
Service fee revenue	(11.0)	(10.3)
Corporate Elimination | Managing general underwriters
Segment information
Unallocated LAE	4.1
Other underwriting expenses	2.7	8.4
Global Property | Reportable segments
Segment information
Gross written premiums	330.7	346.6
Net written premiums	241.3	247.2
Net earned insurance and reinsurance premiums	139.7	136.1
Loss and allocated LAE	(62.6)	(70.4)
Insurance and reinsurance acquisition expenses	(25.8)	(29.3)
Technical profit (loss)	51.3	36.4
Unallocated LAE	(2.1)	(1.9)
Other underwriting expenses	(16.2)	(17.4)
Underwriting (loss) income	33.0	17.1
Underwriting (loss) income, including net service fee income	$ 33.0	$ 17.1
Underwriting Ratios
Loss ratio	46.30%	53.10%
Acquisition expense ratio	18.50%	21.50%
Other underwriting expense ratio	11.60%	12.80%
Combined ratio	76.40%	87.40%
Global A&H | Reportable segments
Segment information
Gross written premiums	$ 169.3	$ 145.6
Net written premiums	134.9	115.5
Net earned insurance and reinsurance premiums	96.1	88.0
Loss and allocated LAE	(63.2)	(45.8)
Insurance and reinsurance acquisition expenses	(26.6)	(29.2)
Technical profit (loss)	6.3	13.0
Unallocated LAE	(1.5)	(1.6)
Other underwriting expenses	(6.1)	(8.0)
Underwriting (loss) income	(1.3)	3.4
Service fee revenue	36.3	32.8
Underwriting (loss) income, including net service fee income	$ 12.0	$ 18.3
Underwriting Ratios
Loss ratio	67.30%	53.90%
Acquisition expense ratio	27.70%	33.20%
Other underwriting expense ratio	6.30%	9.10%
Combined ratio	101.30%	96.20%
Goodwill and intangible assets	$ 584.2	$ 608.6
Global A&H | Reportable segments | Managing general underwriters
Segment information
Unallocated LAE	(4.1)
Other underwriting expenses	(2.7)	(8.4)
General and administrative expenses	$ 16.2	9.5
Specialty & Casualty | Reportable segments
Segment information
Number of core products | product	4
Gross written premiums	$ 120.9	115.5
Net written premiums	108.2	100.6
Net earned insurance and reinsurance premiums	75.7	54.4
Loss and allocated LAE	(47.6)	(21.6)
Insurance and reinsurance acquisition expenses	(20.5)	(14.1)
Technical profit (loss)	7.6	18.7
Unallocated LAE	(1.9)	(1.2)
Other underwriting expenses	(8.2)	(8.0)
Underwriting (loss) income	(2.5)	9.5
Underwriting (loss) income, including net service fee income	$ (2.5)	$ 9.5
Underwriting Ratios
Loss ratio	65.40%	41.90%
Acquisition expense ratio	27.10%	25.90%
Other underwriting expense ratio	10.80%	14.70%
Combined ratio	103.30%	82.50%
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	$ 1.4	$ 7.5
Net written premiums	0.4	6.1
Net earned insurance and reinsurance premiums	0.4	6.0
Loss and allocated LAE	(1.1)	2.4
Insurance and reinsurance acquisition expenses	(0.7)	(0.7)
Technical profit (loss)	(1.4)	7.7
Unallocated LAE	(0.5)	(0.9)
Other underwriting expenses	(2.1)	(1.4)
Underwriting (loss) income	(4.0)	5.4
Underwriting (loss) income, including net service fee income	(4.8)	4.3
Underwriting Ratios
Goodwill and intangible assets	8.1	5.0
Runoff & Other | Reportable segments | Managing general underwriters
Segment information
General and administrative expenses	$ 0.8	$ 1.1