Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment reporting information
Premiums Earned, Net	$ 281.3	$ 284.9	$ 838.1	$ 896.0
Net investment income	22.5	16.8	59.6	42.6
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	26.4	(43.9)	84.0	(33.9)
Other Income	34.8	38.3	110.8	114.4
Total revenues	365.0	296.1	1,092.5	1,019.1
Loss and loss adjustment expenses	165.0	170.0	508.1	534.1
Insurance and Reinsurance Acquisition Expenses	56.4	55.3	159.2	166.1
Other Underwriting Expense	49.6	56.5	156.0	165.5
General and administrative expenses	70.2	97.3	230.4	231.6
Amortization of Intangible Assets	3.0	2.6	9.4	7.9
Interest expense	3.8	3.9	12.4	10.8
Total expenses	345.0	383.0	1,066.1	1,108.1
Pre-tax (loss) income	20.0	(86.9)	26.4	(89.0)
OneBeacon [Member]
Segment reporting information
Premiums Earned, Net	277.9	281.4	827.9	887.3
Net investment income	11.8	12.4	38.3	33.0
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	15.5	(29.9)	56.8	(29.7)
Other Income	1.8	3.9	3.5	(0.4)
Total revenues	307.0	267.8	926.5	890.2
Loss and loss adjustment expenses	162.8	167.5	501.3	527.9
Insurance and Reinsurance Acquisition Expenses	55.1	53.8	154.8	161.2
Other Underwriting Expense	49.4	56.4	155.6	165.2
Interest expense	3.3	3.2	9.8	9.7
Total expenses	274.1	284.6	832.4	876.0
Pre-tax (loss) income	32.9	(16.8)	94.1	14.2
HG Global
Segment reporting information
Premiums Earned, Net	1.2	0.6	3.1	1.7
Net investment income	0.6	0.5	1.6	1.4
Net investment income (loss) - surplus note interest	4.5	4.0	13.4	11.9
Net realized and unrealized investment gains (losses)	(0.3)	0.6	2.3	0.3
Other Income	0.0	0.0	0.0	0.0
Total revenues	6.0	5.7	20.4	15.3
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.2	0.1	0.6	0.3
Other Underwriting Expense	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Total expenses	0.8	0.5	2.0	1.5
Pre-tax (loss) income	5.2	5.2	18.4	13.8
BAM
Segment reporting information
Premiums Earned, Net	0.3	0.3	1.0	0.6
Net investment income	1.7	1.1	5.1	2.9
Net investment income (loss) - surplus note interest	(4.5)	(4.0)	(13.4)	(11.9)
Net realized and unrealized investment gains (losses)	(1.6)	1.7	6.5	2.8
Other Income	0.4	0.2	0.8	0.5
Total revenues	(3.7)	(0.7)	0.0	(5.1)
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.6	0.4	1.9	1.8
Other Underwriting Expense	0.1	0.1	0.3	0.3
Interest expense	0.0	0.0	0.0	0.0
Total expenses	10.0	9.9	30.3	28.4
Pre-tax (loss) income	(13.7)	(10.6)	(30.3)	(33.5)
Other Operations
Segment reporting information
Premiums Earned, Net	1.9	2.6	6.1	6.4
Net investment income	8.4	2.8	14.6	5.3
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	12.8	(16.3)	18.4	(7.3)
Other Income	32.6	34.2	106.5	114.3
Total revenues	55.7	23.3	145.6	118.7
Loss and loss adjustment expenses	2.2	2.5	6.8	6.2
Insurance and Reinsurance Acquisition Expenses	0.5	1.0	1.9	2.8
Other Underwriting Expense	0.1	0.0	0.1	0.0
Interest expense	0.5	0.7	2.6	1.1
Total expenses	60.1	88.0	201.4	202.2
Pre-tax (loss) income	(4.4)	(64.7)	(55.8)	(83.5)
Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses	67.2	94.7	221.0	223.7
Segment Reconciling Items [Member] | OneBeacon [Member]
Segment reporting information
General and administrative expenses	3.2	3.4	10.0	11.0
Segment Reconciling Items [Member] | HG Global
Segment reporting information
General and administrative expenses	0.6	0.4	1.4	1.2
Segment Reconciling Items [Member] | BAM
Segment reporting information
General and administrative expenses	9.3	9.4	28.1	26.3
Segment Reconciling Items [Member] | Other Operations
Segment reporting information
General and administrative expenses	54.1	81.5	181.5	185.2
General and Administrative Expense [Member]
Segment reporting information
Amortization of Intangible Assets	3.0	2.6	9.4	7.9
General and Administrative Expense [Member] | OneBeacon [Member]
Segment reporting information
Amortization of Intangible Assets	0.3	0.3	0.9	1.0
General and Administrative Expense [Member] | HG Global
Segment reporting information
Amortization of Intangible Assets	0.0	0.0	0.0	0.0
General and Administrative Expense [Member] | BAM
Segment reporting information
Amortization of Intangible Assets	0.0	0.0	0.0	0.0
General and Administrative Expense [Member] | Other Operations
Segment reporting information
Amortization of Intangible Assets	2.7	2.3	8.5	6.9
Star & Shield Insurance Exchange
Segment reporting information
Other Income			(21.0)
MediaAlpha [Member] | Other Operations
Segment reporting information
Other Income	27.6	28.3	88.4	81.2
General and administrative expenses	$ 26.2	$ 26.2	$ 82.7	$ 75.8