Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment reporting information
Premiums Earned, Net	$ 274.7	$ 321.8	$ 556.8	$ 611.1
Net investment income	19.2	13.2	37.1	25.8
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	28.1	(15.0)	57.6	10.0
Other Income	36.2	48.7	76.0	76.1
Total revenues	358.2	368.7	727.5	723.0
Loss and loss adjustment expenses	182.0	196.2	343.1	364.1
Insurance and Reinsurance Acquisition Expenses	50.1	57.8	102.8	110.8
Other Underwriting Expense	51.0	53.0	106.4	109.0
General and administrative expenses	73.1	63.4	160.2	134.3
Amortization of Intangible Assets	3.3	2.7	6.4	5.3
Interest expense	4.1	3.4	8.6	6.9
Total expenses	360.3	373.8	721.1	725.1
Pre-tax (loss) income	(2.1)	(5.1)	6.4	(2.1)
OneBeacon [Member]
Segment reporting information
Premiums Earned, Net	271.4	319.3	550.0	605.9
Net investment income	12.1	10.1	26.5	20.6
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	24.7	(14.0)	41.3	0.2
Other Income	0.8	(1.2)	1.7	(4.3)
Total revenues	309.0	314.2	619.5	622.4
Loss and loss adjustment expenses	179.7	194.5	338.5	360.4
Insurance and Reinsurance Acquisition Expenses	48.7	56.4	99.7	107.4
Other Underwriting Expense	50.9	52.9	106.2	108.8
Interest expense	3.2	3.3	6.5	6.5
Total expenses	286.0	311.3	558.3	591.4
Pre-tax (loss) income	23.0	2.9	61.2	31.0
HG Global
Segment reporting information
Premiums Earned, Net	1.0	0.6	1.9	1.1
Net investment income	0.5	0.5	1.0	0.9
Net investment income (loss) - surplus note interest	4.4	3.9	8.9	7.9
Net realized and unrealized investment gains (losses)	0.5	(1.4)	2.6	(0.3)
Other Income	0.0	0.0	0.0	0.0
Total revenues	6.4	3.6	14.4	9.6
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.2	0.1	0.4	0.2
Other Underwriting Expense	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Total expenses	0.5	0.5	1.2	1.0
Pre-tax (loss) income	5.9	3.1	13.2	8.6
BAM
Segment reporting information
Premiums Earned, Net	0.4	0.1	0.7	0.3
Net investment income	1.8	0.8	3.4	1.8
Net investment income (loss) - surplus note interest	(4.4)	(3.9)	(8.9)	(7.9)
Net realized and unrealized investment gains (losses)	3.2	(1.9)	8.1	1.1
Other Income	0.3	0.2	0.4	0.3
Total revenues	1.3	(4.7)	3.7	(4.4)
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.6	0.6	1.3	1.4
Other Underwriting Expense	0.1	0.1	0.2	0.2
Interest expense	0.0	0.0	0.0	0.0
Total expenses	10.3	9.4	20.3	18.5
Pre-tax (loss) income	(9.0)	(14.1)	(16.6)	(22.9)
Other Operations
Segment reporting information
Premiums Earned, Net	1.9	1.8	4.2	3.8
Net investment income	4.8	1.8	6.2	2.5
Net investment income (loss) - surplus note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	(0.3)	2.3	5.6	9.0
Other Income	35.1	49.7	73.9	80.1
Total revenues	41.5	55.6	89.9	95.4
Loss and loss adjustment expenses	2.3	1.7	4.6	3.7
Insurance and Reinsurance Acquisition Expenses	0.6	0.7	1.4	1.8
Other Underwriting Expense	0.0	0.0	0.0	0.0
Interest expense	0.9	0.1	2.1	0.4
Total expenses	63.5	52.6	141.3	114.2
Pre-tax (loss) income	(22.0)	3.0	(51.4)	(18.8)
Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses	69.8	60.7	153.8	129.0
Segment Reconciling Items [Member] | OneBeacon [Member]
Segment reporting information
General and administrative expenses	3.2	3.8	6.8	7.6
Segment Reconciling Items [Member] | HG Global
Segment reporting information
General and administrative expenses	0.3	0.4	0.8	0.8
Segment Reconciling Items [Member] | BAM
Segment reporting information
General and administrative expenses	9.6	8.7	18.8	16.9
Segment Reconciling Items [Member] | Other Operations
Segment reporting information
General and administrative expenses	56.7	47.8	127.4	103.7
General and Administrative Expense [Member]
Segment reporting information
Amortization of Intangible Assets	3.3	2.7	6.4	5.3
General and Administrative Expense [Member] | OneBeacon [Member]
Segment reporting information
Amortization of Intangible Assets	0.3	0.4	0.6	0.7
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	3.0	2.3	5.8	4.6
Star & Shield Insurance Exchange
Segment reporting information
Other Income			(20.0)
MediaAlpha [Member] | Other Operations
Segment reporting information
Other Income	28.1	23.2	0.0	0.0
General and administrative expenses	$ 26.0	$ 21.9	$ 56.5	$ 49.6