Third Party Reinsurance (Written and Earned Premiums) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Written premiums:
Direct	$ 1,325,700,000	$ 1,296,300,000	$ 1,116,700,000
Assumed	36,000,000	65,900,000	59,800,000
Gross written premiums	1,361,700,000	1,362,200,000	1,176,500,000
Ceded	(189,100,000)	(123,200,000)	(74,300,000)
Net written premiums	1,172,600,000	1,239,000,000	1,102,200,000
Earned premiums:
Gross amount	1,322,000,000	1,233,500,000	1,043,800,000
Assumed from other companies	45,900,000	70,900,000	148,500,000
Gross earned premiums	1,367,900,000	1,304,400,000	1,192,300,000
Ceded	(179,700,000)	(119,400,000)	(71,400,000)
Net earned premiums	1,188,200,000	1,185,000,000	1,120,900,000
Losses and LAE:
Direct	802,500,000	802,800,000	584,900,000
Policyholder Benefits and Claims Incurred, Assumed	55,700,000	115,700,000	76,300,000
Gross losses and LAE	858,200,000	918,500,000	661,200,000
Ceded	(149,300,000)	(94,500,000)	(39,100,000)
Total incurred losses and LAE	708,900,000	824,000,000	622,100,000
OneBeacon
Written premiums:
Direct	1,279,900,000	1,257,500,000	1,103,100,000
Assumed	36,000,000	65,900,000	59,800,000
Gross written premiums	1,315,900,000	1,323,400,000	1,162,900,000
Ceded	(179,300,000)	(106,500,000)	(74,300,000)
Net written premiums	1,136,600,000	1,216,900,000	1,088,600,000
Earned premiums:
Gross amount	1,298,000,000	1,209,100,000	1,043,300,000
Assumed from other companies	45,900,000	70,900,000	148,500,000
Gross earned premiums	1,343,900,000	1,280,000,000	1,191,800,000
Ceded	(167,700,000)	(102,900,000)	(71,400,000)
Net earned premiums	1,176,200,000	1,177,100,000	1,120,400,000
Losses and LAE:
Direct	783,000,000	778,700,000	584,900,000
Policyholder Benefits and Claims Incurred, Assumed	55,700,000	115,700,000	76,300,000
Gross losses and LAE	838,700,000	894,400,000	661,200,000
Ceded	(138,000,000)	(79,300,000)	(39,100,000)
Total incurred losses and LAE	700,700,000	815,100,000	622,100,000
HG/BAM
Written premiums:
Direct	25,900,000	16,200,000	13,600,000
Assumed	0	0	0
Gross written premiums	25,900,000	16,200,000	13,600,000
Ceded	0	0	0
Net written premiums	25,900,000	16,200,000	13,600,000
Earned premiums:
Gross amount	3,300,000	1,800,000	500,000
Assumed from other companies	0	0	0
Gross earned premiums	3,300,000	1,800,000	500,000
Ceded	0	0	0
Net earned premiums	3,300,000	1,800,000	500,000
Losses and LAE:
Direct	0	0	0
Policyholder Benefits and Claims Incurred, Assumed	0	0	0
Gross losses and LAE	0	0	0
Ceded	0	0	0
Total incurred losses and LAE	0	0	0
HG/BAM [Member]
Written premiums:
Ceded	(19,300,000)	(12,300,000)	(10,600,000)
Earned premiums:
Ceded	(16,000,000)	(1,400,000)	(10,200,000.0)
Star & Shield Insurance Exchange [Member]
Written premiums:
Ceded		(16,000,000)
Earned premiums:
Ceded		(15,700,000)
Other Segments
Written premiums:
Direct	19,900,000	22,600,000
Assumed	0	0
Gross written premiums	19,900,000	22,600,000
Ceded	(9,800,000)	(16,700,000)
Net written premiums	10,100,000	5,900,000
Earned premiums:
Gross amount	20,700,000	22,600,000
Assumed from other companies	0	0
Gross earned premiums	20,700,000	22,600,000
Ceded	(12,000,000)	(16,500,000)
Net earned premiums	8,700,000	6,100,000	0
Losses and LAE:
Direct	19,500,000	24,100,000
Policyholder Benefits and Claims Incurred, Assumed	0	0
Gross losses and LAE	19,500,000	24,100,000
Ceded	(11,300,000)	(15,200,000)
Total incurred losses and LAE	8,200,000	8,900,000	$ 0
AMTrust [Member]
Written premiums:
Ceded	(33,300,000)
Earned premiums:
Ceded	(33,300,000)
Losses and LAE:
Ceded	$ 33,400,000
Percentage of Quota Share Reinsurance Agreement	100.00%
Star & Shield Insurance Exchange [Member]
Losses and LAE:
Ceded		$ (16,900,000)