Held for Sale and Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions		3 Months Ended			12 Months Ended
	Sep. 28, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		$ 3.0		$ 3.0
Net investment income		19.7		12.8
Net realized and unrealized investment gains		(53.7)		36.3
Total revenues		42.1		88.8
Loss and loss adjustment expenses				1.1
Insurance acquisition expenses		1.4		1.3
Other underwriting expenses		0.1		0.1
General and administrative expenses		47.9		60.7
Interest expense		0.6		0.4
Total expenses		108.1		92.4
Net (loss) income from discontinued operations, net of tax		0.0		32.3
Gain from sale of other discontinued operations, net of tax		0.1	$ 0.1	(1.0)
Total income (loss) from discontinued operations		0.1		31.3
Total comprehensive income (loss)		(48.0)		29.2
Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax					$ 363.2
OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) income from discontinued operations, net of tax		32.3
Gain from sale of other discontinued operations, net of tax	$ 554.6
Tranzact
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 income				0.0
Income tax expense				0.0	30.2
Net (loss) income from discontinued operations, net of tax				0.0
Gain from sale of other discontinued operations, net of tax					$ 51.9
Total comprehensive income (loss)				(1.0)
Discontinued Operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total income (loss) from discontinued operations		0.1		31.3
Discontinued Operations | Sirius Group
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 income		0.0
Income tax expense		0.0
Net (loss) income from discontinued operations, net of tax		0.0
Gain from sale of other discontinued operations, net of tax		0.1
Total income (loss) from discontinued operations		0.1
Total comprehensive income (loss)		0.1
Discontinued Operations | Sirius Group | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax		0.1
Discontinued Operations | OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums				261.8
Net investment income				12.2
Net realized and unrealized investment gains				15.0
Other revenue				3.4
Total revenues				292.4
Loss and loss adjustment expenses				150.6
Insurance acquisition expenses				45.3
Other underwriting expenses				51.7
General and administrative expenses				5.0
Interest expense				3.3
Total expenses				255.9
Pre-tax income				36.5
Income tax expense				(4.2)
Net (loss) income from discontinued operations, net of tax				32.3
Total income (loss) from discontinued operations				32.3
Net change in foreign currency translation and benefit plan assets and obligations				(0.1)
Total comprehensive income (loss)				32.2
Discontinued Operations | Total
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		0.0		261.8
Net investment income		0.0		12.2
Net realized and unrealized investment gains		0.0		15.0
Other revenue		0.0		3.4
Total revenues		0.0		292.4
Loss and loss adjustment expenses		0.0		150.6
Insurance acquisition expenses		0.0		45.3
Other underwriting expenses		0.0		51.7
General and administrative expenses		0.0		5.0
Interest expense		0.0		3.3
Total expenses		0.0		255.9
Pre-tax income		0.0		36.5
Income tax expense		0.0		(4.2)
Net (loss) income from discontinued operations, net of tax		0.0		32.3
Total income (loss) from discontinued operations		0.1		31.3
Net change in foreign currency translation and benefit plan assets and obligations		0.0		(0.1)
Total comprehensive income (loss)		0.1		31.2
Discontinued Operations | Total | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax		$ 0.1		(1.0)
Discontinued Operations | Tranzact
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax				(1.0)
Total income (loss) from discontinued operations				(1.0)
Discontinued Operations | Tranzact | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain from sale of other discontinued operations, net of tax				$ (1.0)