Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment reporting information
Earned insurance premiums	$ 282.1	$ 289.3
Net investment income	17.9	12.6
Net investment income (loss) - surplus note interest	0.0	0.0
Net realized and unrealized investment gains (losses)	29.5	25.0
Other revenue	97.6	64.9
Total revenues	427.1	391.8
Losses and LAE	161.1	167.9
Insurance and Reinsurance Acquisition Expenses	52.7	53.0
Other Underwriting Expense	55.4	56.0
General and administrative expenses	140.5	110.1
Amortization of Intangible Assets	8.1	6.9
Interest expense	5.9	4.3
Total expenses	415.6	391.3
Pre-tax (loss) income	11.5	0.5
OneBeacon [Member]
Segment reporting information
Earned insurance premiums	278.6	286.6
Net investment income	14.4	10.5
Net investment income (loss) - surplus note interest	0.0	0.0
Net realized and unrealized investment gains (losses)	16.6	14.2
Other revenue	0.9	(3.1)
Total revenues	310.5	308.2
Losses and LAE	158.8	165.9
Insurance and Reinsurance Acquisition Expenses	51.0	51.0
Other Underwriting Expense	55.3	55.9
Interest expense	3.3	3.2
Total expenses	272.3	280.1
Pre-tax (loss) income	38.2	28.1
HG Global
Segment reporting information
Earned insurance premiums	0.9	0.5
Net investment income	0.5	0.4
Net investment income (loss) - surplus note interest	4.5
Net realized and unrealized investment gains (losses)	2.1	1.1
Other revenue	0.0	0.0
Total revenues	8.0	6.0
Losses and LAE	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.2	0.1
Other Underwriting Expense	0.0	0.0
Interest expense	0.0	0.0
Total expenses	0.7	0.5
Pre-tax (loss) income	7.3
BAM
Segment reporting information
Earned insurance premiums	0.3	0.2
Net investment income	1.6	1.0
Net investment income (loss) - surplus note interest	(4.5)	(4.0)
Net realized and unrealized investment gains (losses)	4.9	3.0
Other revenue	0.1	0.1
Total revenues	2.4	0.3
Losses and LAE	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.7	0.8
Other Underwriting Expense	0.1	0.1
Interest expense	0.0	0.0
Total expenses	10.0	9.1
Pre-tax (loss) income	(7.6)	(8.8)
Other Operations
Segment reporting information
Earned insurance premiums	2.3	2.0
Net investment income	1.4	0.7
Net investment income (loss) - surplus note interest	0.0	0.0
Net realized and unrealized investment gains (losses)	5.9	6.7
Other revenue	96.6	67.9
Total revenues	106.2	77.3
Losses and LAE	2.3	2.0
Insurance and Reinsurance Acquisition Expenses	0.8	1.1
Other Underwriting Expense	0.0	0.0
Interest expense	2.6	1.1
Total expenses	132.6	101.6
Pre-tax (loss) income	(26.4)	(24.3)
Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses	132.4	103.2
Segment Reconciling Items [Member] | OneBeacon [Member]
Segment reporting information
General and administrative expenses	3.6	3.8
Segment Reconciling Items [Member] | HG Global
Segment reporting information
General and administrative expenses	0.5	0.4
Segment Reconciling Items [Member] | BAM
Segment reporting information
General and administrative expenses	9.2	8.2
Segment Reconciling Items [Member] | Other Operations
Segment reporting information
General and administrative expenses	119.1	90.8
General and Administrative Expense [Member]
Segment reporting information
Amortization of Intangible Assets	8.1	6.9
General and Administrative Expense [Member] | OneBeacon [Member]
Segment reporting information
Amortization of Intangible Assets	0.3	0.3
General and Administrative Expense [Member] | HG Global
Segment reporting information
Amortization of Intangible Assets	0.0	0.0
General and Administrative Expense [Member] | BAM
Segment reporting information
Amortization of Intangible Assets	0.0	0.0
General and Administrative Expense [Member] | Other Operations
Segment reporting information
Amortization of Intangible Assets	7.8	6.6
Tranzact [Member] | Other Operations
Segment reporting information
Other revenue	57.8	37.5
General and administrative expenses	48.4	34.9
MediaAlpha [Member] | Other Operations
Segment reporting information
Other revenue	32.7	29.7
General and administrative expenses	$ 30.5	$ 27.7