Third Party Reinsurance (Written and Earned Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Written premiums:
Direct	$ 1,245.2	$ 1,325.7	$ 1,296.3
Assumed	28.0	36.0	65.9
Gross written premiums	1,273.2	1,361.7	1,362.2
Ceded	(127.4)	(189.1)	(123.2)
Net written premiums	1,145.8	1,172.6	1,239.0
Earned premiums:
Direct Premiums amount	1,198.1	1,322.0	1,233.5
Assumed	29.4	45.9	70.9
Gross earned premiums	1,227.5	1,367.9	1,304.4
Ceded	(113.5)	(179.7)	(119.4)
Net earned premiums	1,114.0	1,188.2	1,185.0
Losses and LAE:
Direct	693.8	802.5	802.8
Assumed	21.2	55.7	115.7
Gross losses and LAE	715.0	858.2	918.5
Ceded	(51.0)	(149.3)	(94.5)
Total incurred losses and LAE	664.0	708.9	824.0
OneBeacon
Written premiums:
Direct	1,193.3	1,279.9	1,257.5
Assumed	28.0	36.0	65.9
Gross written premiums	1,221.3	1,315.9	1,323.4
Ceded	(120.6)	(179.3)	(106.5)
Net written premiums	1,100.7	1,136.6	1,216.9
Earned premiums:
Direct Premiums amount	1,177.0	1,298.0	1,209.1
Assumed	29.4	45.9	70.9
Gross earned premiums	1,206.4	1,343.9	1,280.0
Ceded	(105.8)	(167.7)	(102.9)
Net earned premiums	1,100.6	1,176.2	1,177.1
Losses and LAE:
Direct	679.5	783.0	778.7
Assumed	21.2	55.7	115.7
Gross losses and LAE	700.7	838.7	894.4
Ceded	(44.7)	(138.0)	(79.3)
Total incurred losses and LAE	656.0	700.7	815.1
HG/BAM
Written premiums:
Direct	38.6	25.9	16.2
Assumed	0.0	0.0	0.0
Gross written premiums	38.6	25.9	16.2
Ceded	0.0	0.0	0.0
Net written premiums	38.6	25.9	16.2
Earned premiums:
Direct Premiums amount	5.9	3.3	1.8
Assumed	0.0	0.0	0.0
Gross earned premiums	5.9	3.3	1.8
Ceded	0.0	0.0	0.0
Net earned premiums	5.9	3.3	1.8
Losses and LAE:
Direct	0.0	0.0	0.0
Assumed	0.0	0.0	0.0
Gross losses and LAE	0.0	0.0	0.0
Ceded	0.0	0.0	0.0
Total incurred losses and LAE	0.0	0.0	0.0
HG/BAM [Member]
Written premiums:
Ceded	27.2	19.3	12.3
Earned premiums:
Ceded	21.0	16.0	1.4
Other Segments
Written premiums:
Direct	13.3	19.9	22.6
Assumed	0.0	0.0	0.0
Gross written premiums	13.3	19.9	22.6
Ceded	(6.8)	(9.8)	(16.7)
Net written premiums	6.5	10.1	5.9
Earned premiums:
Direct Premiums amount	15.2	20.7	22.6
Assumed	0.0	0.0	0.0
Gross earned premiums	15.2	20.7	22.6
Ceded	(7.7)	(12.0)	(16.5)
Net earned premiums	7.5	8.7	6.1
Losses and LAE:
Direct	14.3	19.5	24.1
Assumed	0.0	0.0	0.0
Gross losses and LAE	14.3	19.5	24.1
Ceded	(6.3)	(11.3)	(15.2)
Total incurred losses and LAE	$ 8.0	8.2	8.9
AMTrust [Member]
Written premiums:
Ceded		33.3
Earned premiums:
Ceded		33.3
Losses and LAE:
Ceded		$ 33.4
Percentage of quota share reinsurance agreement		100.00%
Star & Shield Insurance Exchange [Member]
Written premiums:
Ceded			16.0
Earned premiums:
Ceded			15.7
Losses and LAE:
Ceded			$ 16.9