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Significant transactions - IMG Acquisition (Details) - USD ($) $ in Millions			3 Months Ended								12 Months Ended
	Nov. 08, 2018	May 26, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liabilities assumed
Goodwill			$ 400.6				$ 401.0				$ 400.6	$ 401.0
Payment of contingent consideration											4.4	30.6
Gain on revaluation of contingent consideration											9.6	48.8
Gross written premiums											1,821.0	1,439.3	$ 1,269.0
Revenues			336.8	$ 351.7	$ 441.8	$ 327.5	$ 362.6	$ 293.0	$ 252.6	$ 225.9	1,457.8	1,134.1	994.5
Net income											(18.1)	(150.0)	$ 32.5
IMG and its subsidiaries
Business combination
Percentage of ownership interest acquired		100.00%
Earnout		$ 50.0
Purchase price
Cash paid		250.8
Series A redeemable preference shares		100.0
Contingent consideration		43.1	$ 19.9								19.9
Total enterprise value		393.9
Less: Debt assumed		(129.5)
Total purchase price		264.4
Assets acquired
Total investments		41.0
Cash		10.5
Accrued investment income		0.2
Insurance and reinsurance premiums receivable		1.6
Deferred acquisition costs		2.9
Other assets		12.9
Total assets acquired		214.1
Liabilities assumed
Loss and loss adjustment expense reserves		14.3
Unearned insurance and reinsurance premiums		9.8
Deferred tax liability		55.9
Debt		129.5
Other liabilities		18.2
Total liabilities assumed		227.7
Net assets acquired		(13.6)
Goodwill		278.0
Goodwill recognized is expected to be deductible for income tax purposes		0.0
Payment of contingent consideration	$ 3.5
Gain on revaluation of contingent consideration											6.1	$ 13.6
Gross written premiums											$ 136.0
Gross written premiums (in percent)											7.00%	11.00%	14.00%
Revenues												$ 79.6
Net income												$ 13.3
IMG and its subsidiaries \| Trade Names
Assets acquired
Intangible asset-trade names		27.0
IMG and its subsidiaries \| Distribution relationships
Assets acquired
Intangible asset		$ 91.0
Intangible asset useful life		13 years
IMG and its subsidiaries \| Customer relationships
Assets acquired
Intangible asset		$ 17.0
Intangible asset useful life		12 years 6 months
IMG and its subsidiaries \| Technology
Assets acquired
Intangible asset		$ 10.0
Intangible asset useful life		5 years
IMG and its subsidiaries \| Certus
Business combination
Secured promissory note issued		$ 9.0