Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment reporting information
Premiums Earned, Net	$ 264.8	$ 282.1
Net investment income	26.1	17.9
Net investment income (loss) - surplus note interest	0.0	0.0
Net realized and unrealized investment gains	51.3	29.5
Other Income	43.7	39.8
Total revenues	385.9	369.3
Loss and loss adjustment expenses	151.7	161.1
Insurance Acquisition Expenses	46.6	52.7
Other Underwriting Expense	51.8	55.4
General and administrative expenses	94.2	87.1
Amortization of Intangible Assets	2.9	3.1
Interest expense	3.7	4.5
Total expenses	348.0	360.8
Pre-tax (loss) income	37.9	8.5
OneBeacon [Member]
Segment reporting information
Premiums Earned, Net	261.8	278.6
Net investment income	12.2	14.4
Net investment income (loss) - surplus note interest	0.0	0.0
Net realized and unrealized investment gains	15.0	16.6
Other Income	3.4	0.9
Total revenues	292.4	310.5
Loss and loss adjustment expenses	150.6	158.8
Insurance Acquisition Expenses	45.3	51.0
Other Underwriting Expense	51.7	55.3
Interest expense	3.3	3.3
Total expenses	255.9	272.3
Pre-tax (loss) income	36.5	38.2
HG Global
Segment reporting information
Premiums Earned, Net	1.5	0.9
Net investment income	0.6	0.5
Net investment income (loss) - surplus note interest	4.8	4.5
Net realized and unrealized investment gains	0.3	2.1
Other Income	0.0	0.0
Total revenues	7.2	8.0
Loss and loss adjustment expenses	0.0	0.0
Insurance Acquisition Expenses	0.3	0.2
Other Underwriting Expense	0.0	0.0
Interest expense	0.0	0.0
Total expenses	0.6	0.7
Pre-tax (loss) income	6.6	7.3
BAM
Segment reporting information
Premiums Earned, Net	0.5	0.3
Net investment income	2.0	1.6
Net investment income (loss) - surplus note interest	(4.8)	(4.5)
Net realized and unrealized investment gains	1.0	4.9
Other Income	0.4	0.1
Total revenues	(0.9)	2.4
Loss and loss adjustment expenses	0.0	0.0
Insurance Acquisition Expenses	0.9	0.7
Other Underwriting Expense	0.1	0.1
Interest expense	0.0	0.0
Total expenses	11.3	10.0
Pre-tax (loss) income	(12.2)	(7.6)
Other Operations
Segment reporting information
Premiums Earned, Net	1.0	2.3
Net investment income	11.3	1.4
Net investment income (loss) - surplus note interest	0.0	0.0
Net realized and unrealized investment gains	35.0	5.9
Other Income	39.9	38.8
Total revenues	87.2	48.4
Loss and loss adjustment expenses	1.1	2.3
Insurance Acquisition Expenses	0.1	0.8
Other Underwriting Expense	0.0	0.0
Interest expense	0.4	1.2
Total expenses	80.2	77.8
Pre-tax (loss) income	7.0	(29.4)
Segment Reconciling Items [Member]
Segment reporting information
General and administrative expenses	91.3	84.0
Segment Reconciling Items [Member] | OneBeacon [Member]
Segment reporting information
General and administrative expenses	4.7	3.6
Segment Reconciling Items [Member] | HG Global
Segment reporting information
General and administrative expenses	0.3	0.5
Segment Reconciling Items [Member] | BAM
Segment reporting information
General and administrative expenses	10.3	9.2
Segment Reconciling Items [Member] | Other Operations
Segment reporting information
General and administrative expenses	76.0	70.7
General and Administrative Expense [Member]
Segment reporting information
Amortization of Intangible Assets	2.9	3.1
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	2.6	2.8
MediaAlpha [Member] | Other Operations
Segment reporting information
Other Income	32.5	32.7
General and administrative expenses	$ 30.9	$ 30.5