Held for Sale and Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Sep. 28, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		$ 3.4	$ 2.2	$ 6.4	$ 5.2
Net investment income		11.8	14.7	31.5	27.5
Net realized and unrealized investment gains		10.5	33.7	(43.2)	70.0
Total revenues		122.3	83.5	164.4	172.3
Loss and loss adjustment expenses					1.1
Insurance acquisition expenses		1.3	0.9	2.7	2.1
Other underwriting expenses		0.1	0.1	0.2	0.2
General and administrative expenses		61.1	54.9	106.1	113.3
Interest expense		2.0	0.5	2.6	0.9
Total expenses		134.7	85.7	242.8	178.1
Net (loss) income from discontinued operations, net of tax		0.0	3.4	0.0	35.7
(Loss) gain from sale of discontinued operations, net of tax		0.0	(0.6)	0.1	(1.6)
Total income (loss) from discontinued operations		0.0	2.8	0.1	34.1
Total comprehensive income (loss)		2.6	13.8	(45.4)	43.0
OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) income from discontinued operations, net of tax			3.4		35.7
(Loss) gain from sale of discontinued operations, net of tax	$ 554.6
Other Discontinued Operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total comprehensive income (loss)					(1.0)
Discontinued Operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total income (loss) from discontinued operations		$ 0.0	2.8	0.1	34.1
Discontinued Operations | OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			277.4		539.2
Net investment income			14.5		26.7
Net realized and unrealized investment gains			12.3		27.3
Other revenue			2.1		5.5
Total revenues			306.3		598.7
Loss and loss adjustment expenses			188.6		339.2
Insurance acquisition expenses			48.4		93.7
Other underwriting expenses			59.6		111.3
General and administrative expenses			8.8		13.8
Interest expense			3.3		6.6
Total expenses			308.7		564.6
Pre-tax (loss) income			(2.4)		34.1
Income tax expense			5.8		1.6
Net (loss) income from discontinued operations, net of tax			3.4		35.7
Total income (loss) from discontinued operations			3.4		35.7
Net change in foreign currency translation and other			0.2		0.3
Total comprehensive income (loss)			3.6		36.0
Discontinued Operations | Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			0.0		0.0
Net investment income			0.0		0.0
Net realized and unrealized investment gains			0.0		0.0
Other revenue			0.0		0.0
Total revenues			0.0		0.0
Loss and loss adjustment expenses			0.0		0.0
Insurance acquisition expenses			0.0		0.0
Other underwriting expenses			0.0		0.0
General and administrative expenses			0.0		0.0
Interest expense			0.0		0.0
Total expenses			0.0		0.0
Pre-tax (loss) income			0.0		0.0
Income tax expense			0.0		0.0
Net (loss) income from discontinued operations, net of tax			0.0		0.0
(Loss) gain from sale of discontinued operations, net of tax			(0.6)	$ 0.1	(0.6)
Total income (loss) from discontinued operations			(0.6)		(0.6)
Total comprehensive income (loss)			(0.6)		(0.6)
Discontinued Operations | Sirius Group | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
(Loss) gain from sale of discontinued operations, net of tax			(0.6)		(0.6)
Discontinued Operations | Other Discontinued Operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums					0.0
Net investment income					0.0
Net realized and unrealized investment gains					0.0
Other revenue					0.0
Total revenues					0.0
Loss and loss adjustment expenses					0.0
Insurance acquisition expenses					0.0
Other underwriting expenses					0.0
General and administrative expenses					0.0
Interest expense					0.0
Total expenses					0.0
Pre-tax (loss) income					0.0
Income tax expense					0.0
Net (loss) income from discontinued operations, net of tax					0.0
Total income (loss) from discontinued operations					(1.0)
Discontinued Operations | Other Discontinued Operations | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
(Loss) gain from sale of discontinued operations, net of tax					(1.0)
Discontinued Operations | Total
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			277.4		539.2
Net investment income			14.5		26.7
Net realized and unrealized investment gains			12.3		27.3
Other revenue			2.1		5.5
Total revenues			306.3		598.7
Loss and loss adjustment expenses			188.6		339.2
Insurance acquisition expenses			48.4		93.7
Other underwriting expenses			59.6		111.3
General and administrative expenses			8.8		13.8
Interest expense			3.3		6.6
Total expenses			308.7		564.6
Pre-tax (loss) income			(2.4)		34.1
Income tax expense			5.8		1.6
Net (loss) income from discontinued operations, net of tax			3.4		35.7
Total income (loss) from discontinued operations			2.8		34.1
Net change in foreign currency translation and other			0.2		0.3
Total comprehensive income (loss)			3.0		34.4
Discontinued Operations | Total | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
(Loss) gain from sale of discontinued operations, net of tax			$ (0.6)		$ (1.6)