Reserves for unpaid losses and loss adjustment expenses - Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reserves for unpaid losses and loss adjustment expenses
Asbestos three year net paid survival ratio	10 years 2 months 12 days	10 years 2 months 12 days
Environmental three year net paid survival ratio	4 years 2 months 12 days	4 years 3 months 18 days
Asbestos, Gross:
Beginning balance, Gross	$ 259.2	$ 187.0	$ 193.5
Incurred losses and LAE, Gross	(6.9)	96.9	15.3
Paid losses and LAE, Gross	(15.5)	(24.7)	(21.8)
Ending balance, Gross	236.8	259.2	187.0
Asbestos, Net:
Beginning balance, Net	204.6	166.4	172.9
Incurred losses and LAE, Net	(6.9)	59.0	13.6
Paid losses and LAE, Net	(13.3)	(20.8)	(20.1)
Ending balance, Net	184.4	204.6	166.4
Environmental Claims, Gross:
Beginning balance, Gross	16.7	18.5	21.5
Incurred losses and LAE, Gross	4.0	2.9	0.4
Paid losses and LAE, Gross	(4.0)	(4.7)	(3.4)
Ending balance, Gross	16.7	16.7	18.5
Environmental Claims, Net:
Beginning balance, Net	16.0	13.9	16.8
Incurred losses and LAE, Net	4.0	6.1	0.4
Paid losses and LAE, Net	(4.1)	(4.0)	(3.3)
Ending balance, Net	15.9	16.0	13.9
Total asbestos and environmental, Gross:
Beginning balance, Gross	275.9	205.5	215.0
Incurred losses and LAE, Gross	(3.0)	99.8	15.7
Paid losses and LAE, Gross	(19.5)	(29.4)	(25.2)
Ending balance, Gross	253.4	275.9	205.5
Total asbestos and environmental, Net:
Beginning balance, Net	220.6	180.3	189.7
Incurred losses and LAE, Net	(2.9)	65.1	14.0
Paid losses and LAE, Net	(17.4)	(24.8)	(23.4)
Ending balance, Net	200.3	220.6	180.3
Net loss reserves by type
Case Reserve	924.5	833.9
IBNR Reserve	742.0	744.9
Liability for Unpaid Claims and Claims Adjustment Expense, Net, Total	$ 1,666.5	$ 1,578.8	$ 1,328.6	$ 1,361.3