Deferred acquisition costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Deferred Policy Acquisition Costs, Beginning Balance	$ 120.9	$ 84.7	$ 74.6
Acquisition costs deferred	202.6	220.7	137.7
Amortization expense	(180.4)	(186.7)	(126.7)
Other, including foreign exchange	(1.5)	2.2	(0.9)
Deferred Policy Acquisition Costs, Ending Balance	$ 141.6	120.9	$ 84.7
IMG
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Acquisition costs deferred		$ 2.9