Third Party Reinsurance (Written and Earned Premiums) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Written premiums:
Direct	$ 1,505.0
Assumed	993.8
Gross written premiums	2,498.8
Ceded	377.3
Net written premiums	2,121.5
Earned premiums:
Gross amount	1,433.7
Assumed from other companies	996.3
Gross earned premiums	2,430.0
Ceded	371.1
Net earned premiums	2,058.9	1,987.3	2,063.6
Losses and LAE:
Direct	972.6
Policyholder Benefits and Claims Incurred, Assumed	441.8
Gross losses and LAE	1,414.4
Ceded	(245.1)
Total incurred losses and LAE	1,169.3	1,040.5	1,193.9
OneBeacon
Written premiums:
Direct	1,257.5	1,103.1	1,204.0
Assumed	65.9	59.8	55.2
Gross written premiums	1,323.4	1,162.9	1,259.2
Ceded	106.5	74.3	80.0
Net written premiums	1,216.9	1,088.6	1,179.2
Earned premiums:
Gross amount	1,209.1	1,043.3	1,158.3
Assumed from other companies	70.9	148.5	52.8
Gross earned premiums	1,280.0	1,191.8	1,211.1
Ceded	102.9	71.4	79.1
Net earned premiums	1,177.1	1,120.4	1,132.0
Losses and LAE:
Direct	830.7	584.9	687.5
Policyholder Benefits and Claims Incurred, Assumed	63.7	76.3	29.6
Gross losses and LAE	894.4	661.2	717.1
Ceded	(79.3)	(39.1)	(67.1)
Total incurred losses and LAE	815.1	622.1	650.0
Sirius Group
Written premiums:
Direct	208.7	177.3	186.1
Assumed	927.9	943.1	992.7
Gross written premiums	1,136.6	1,120.4	1,178.8
Ceded	254.1	243.8	231.1
Net written premiums	882.5	876.6	947.7
Earned premiums:
Gross amount	200.2	174.0	169.9
Assumed from other companies	925.4	938.6	988.3
Gross earned premiums	1,125.6	1,112.6	1,158.2
Ceded	251.7	246.2	226.6
Net earned premiums	873.9	866.4	931.6
Losses and LAE:
Direct	117.8	98.1	96.9
Policyholder Benefits and Claims Incurred, Assumed	378.1	455.5	523.9
Gross losses and LAE	495.9	553.6	620.8
Ceded	(150.6)	(135.2)	(76.9)
Total incurred losses and LAE	345.3	418.4	543.9
HG/BAM
Written premiums:
Direct	16.2
Assumed	0
Gross written premiums	16.2
Ceded	0
Net written premiums	16.2
Earned premiums:
Gross amount	1.8
Assumed from other companies	0
Gross earned premiums	1.8
Ceded	0
Net earned premiums	1.8
Losses and LAE:
Direct	0
Policyholder Benefits and Claims Incurred, Assumed	0
Gross losses and LAE	0
Ceded	0
Total incurred losses and LAE	0
HG/BAM [Member]
Written premiums:
Ceded	12.3
Earned premiums:
Ceded	1.4
Star & Shield Insurance Exchange [Member]
Written premiums:
Ceded	16.0
Earned premiums:
Ceded	15.7
Other Segments
Written premiums:
Direct	22.6
Assumed	0
Gross written premiums	22.6
Ceded	16.7
Net written premiums	5.9
Earned premiums:
Gross amount	22.6
Assumed from other companies	0
Gross earned premiums	22.6
Ceded	16.5
Net earned premiums	6.1	0	0
Losses and LAE:
Direct	24.1
Policyholder Benefits and Claims Incurred, Assumed	0
Gross losses and LAE	24.1
Ceded	(15.2)
Total incurred losses and LAE	8.9	0	0
OneBeacon and Sirius Group
Written premiums:
Direct		1,280.4	1,390.1
Assumed		1,002.9	1,047.9
Gross written premiums		2,283.3	2,438.0
Ceded		318.1	311.1
Net written premiums		1,965.2	2,126.9
Earned premiums:
Gross amount		1,217.3	1,328.2
Assumed from other companies		1,087.1	1,041.1
Gross earned premiums		2,304.4	2,369.3
Ceded		317.6	305.7
Net earned premiums		1,986.8	2,063.6
Losses and LAE:
Direct		683.0	784.4
Policyholder Benefits and Claims Incurred, Assumed		531.8	553.5
Gross losses and LAE		1,214.8	1,337.9
Ceded		(174.3)	(144.0)
Total incurred losses and LAE		$ 1,040.5	$ 1,193.9