Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary Insurance Information
Deferred acquisition costs	$ 141.6	$ 120.9	$ 84.7
Loss and loss adjustment expense reserves	2,016.7	1,898.5	1,620.1
Unearned insurance and reinsurance premiums	647.2	506.8	398.0
Net earned insurance and reinsurance premiums	1,262.3	1,035.3	890.1
Loss and Loss Adjustment Expenses	900.0	811.2	519.3
Insurance and reinsurance acquisition costs	255.4	197.2	210.3
Other Underwriting Expenses	146.2	106.1	107.3
Net Written Insurance and Reinsurance Premiums	1,357.1	1,090.2	938.1
Corporate Elimination
Supplementary Insurance Information
Loss and Loss Adjustment Expenses	13.0
Insurance and reinsurance acquisition costs	(43.7)	(42.9)
Other Underwriting Expenses	16.1
Global Property | Reportable segments
Supplementary Insurance Information
Deferred acquisition costs	44.1	43.5	43.5
Loss and loss adjustment expense reserves	910.6	759.6	506.6
Unearned insurance and reinsurance premiums	252.0	195.7	171.3
Net earned insurance and reinsurance premiums	636.0	564.4	481.8
Loss and Loss Adjustment Expenses	528.7	512.4	268.5
Insurance and reinsurance acquisition costs	121.1	112.9	100.3
Other Underwriting Expenses	68.2	63.3	65.1
Net Written Insurance and Reinsurance Premiums	655.0	556.2	514.2
Global A&H | Reportable segments
Supplementary Insurance Information
Deferred acquisition costs	54.0	49.1	24.4
Loss and loss adjustment expense reserves	197.9	206.2	158.3
Unearned insurance and reinsurance premiums	224.8	195.8	122.5
Net earned insurance and reinsurance premiums	357.6	306.8	272.2
Loss and Loss Adjustment Expenses	194.9	179.8	171.6
Insurance and reinsurance acquisition costs	109.7	89.6	71.7
Other Underwriting Expenses	27.4	23.4	23.3
Net Written Insurance and Reinsurance Premiums	379.8	341.5	277.6
Specialty & Casualty | Reportable segments
Supplementary Insurance Information
Deferred acquisition costs	43.5	28.2	18.3
Loss and loss adjustment expense reserves	323.4	304.5	314.0
Unearned insurance and reinsurance premiums	170.4	115.0	85.3
Net earned insurance and reinsurance premiums	239.5	163.2	135.2
Loss and Loss Adjustment Expenses	148.8	104.9	75.6
Insurance and reinsurance acquisition costs	65.5	41.1	37.7
Other Underwriting Expenses	28.2	16.5	15.1
Net Written Insurance and Reinsurance Premiums	292.7	193.0	137.4
Runoff & Other | Reportable segments
Supplementary Insurance Information
Deferred acquisition costs		0.1	(1.5)
Loss and loss adjustment expense reserves	584.8	628.2	641.2
Unearned insurance and reinsurance premiums		0.3	18.9
Net earned insurance and reinsurance premiums	29.2	0.9	0.9
Loss and Loss Adjustment Expenses	14.6	14.1	3.6
Insurance and reinsurance acquisition costs	2.8	(3.5)	0.6
Other Underwriting Expenses	6.3	2.9	3.8
Net Written Insurance and Reinsurance Premiums	$ 29.6	$ (0.5)	$ 8.9