Segment Information (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment reporting information
Net investment income	$ 23,300,000	$ 24,700,000	$ 24,700,000
Earned insurance and reinsurance premiums	494,300,000	493,600,000	493,600,000
Net investment income (loss) - surplus note interest	0		0
Net realized and unrealized investment gains (losses)	103,400,000	63,800,000	63,800,000
Other revenue	35,800,000	(3,400,000)	(3,400,000)
Total revenues	656,800,000	578,700,000	578,700,000
Losses and LAE	265,400,000	229,300,000	229,300,000
Insurance and Reinsurance Acquisition Expenses	95,600,000	95,100,000	95,100,000
Other underwriting expenses	81,500,000	81,400,000	81,400,000
General and administrative expenses	116,400,000	50,300,000	50,300,000
Interest expense	11,100,000	10,100,000	10,100,000
Total expenses	570,000,000	466,200,000	466,200,000
Pre-tax income (loss)	86,800,000	112,500,000	112,500,000
OneBeacon
Segment reporting information
Net investment income	10,500,000		10,400,000
Earned insurance and reinsurance premiums	286,600,000		276,500,000
Net investment income (loss) - surplus note interest	0		0
Net realized and unrealized investment gains (losses)	14,200,000		18,900,000
Other revenue	(3,100,000)		1,000,000
Total revenues	308,200,000		306,800,000
Losses and LAE	165,900,000		149,400,000
Insurance and Reinsurance Acquisition Expenses	51,000,000		46,700,000
Other underwriting expenses	55,900,000		49,400,000
General and administrative expenses	4,100,000		3,300,000
Interest expense	3,200,000		3,200,000
Total expenses	280,100,000		252,000,000
Pre-tax income (loss)	28,100,000		54,800,000
Sirius Group
Segment reporting information
Net investment income	8,000,000		8,700,000
Earned insurance and reinsurance premiums	205,000,000		215,500,000
Net investment income (loss) - surplus note interest	0		0
Net realized and unrealized investment gains (losses)	79,900,000		28,100,000
Other revenue	(27,400,000)		(5,300,000)
Total revenues	265,500,000		247,000,000
Losses and LAE	97,500,000		78,800,000
Insurance and Reinsurance Acquisition Expenses	42,600,000		47,400,000
Other underwriting expenses	25,500,000		31,800,000
General and administrative expenses	6,200,000		8,400,000
Interest expense	6,800,000		6,600,000
Total expenses	178,600,000		173,000,000
Pre-tax income (loss)	86,900,000		74,000,000
HG Global
Segment reporting information
Net investment income	400,000		300,000
Earned insurance and reinsurance premiums	500,000		200,000
Net investment income (loss) - surplus note interest	4,000,000		3,900,000
Net realized and unrealized investment gains (losses)	1,100,000		700,000
Other revenue	0		0
Total revenues	6,000,000		5,100,000
Losses and LAE	0		0
Insurance and Reinsurance Acquisition Expenses	100,000		100,000
Other underwriting expenses	0		0
General and administrative expenses	400,000		400,000
Interest expense	0		0
Total expenses	500,000		500,000
Pre-tax income (loss)	5,500,000		4,600,000
BAM
Segment reporting information
Net investment income	1,000,000		1,400,000
Earned insurance and reinsurance premiums	200,000		100,000
Net investment income (loss) - surplus note interest	(4,000,000)		(3,900,000)
Net realized and unrealized investment gains (losses)	3,000,000		3,000,000
Other revenue	100,000		200,000
Total revenues	300,000		800,000
Losses and LAE	0		0
Insurance and Reinsurance Acquisition Expenses	800,000		400,000
Other underwriting expenses	100,000		100,000
General and administrative expenses	8,200,000		8,900,000
Interest expense	0		0
Total expenses	9,100,000		9,400,000
Pre-tax income (loss)	(8,800,000)		(8,600,000)
Other Operations
Segment reporting information
Net investment income	3,400,000		3,900,000
Earned insurance and reinsurance premiums	2,000,000		1,300,000
Net investment income (loss) - surplus note interest	0		0
Net realized and unrealized investment gains (losses)	5,200,000		13,100,000
Other revenue	66,200,000		700,000
Total revenues	76,800,000		19,000,000
Losses and LAE	2,000,000		1,100,000
Insurance and Reinsurance Acquisition Expenses	1,100,000		500,000
Other underwriting expenses	0		100,000
General and administrative expenses	97,500,000		29,300,000
Interest expense	1,100,000		300,000
Total expenses	101,700,000		31,300,000
Pre-tax income (loss)	$ (24,900,000)		$ (12,300,000)