Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended		12 Months Ended
	Oct. 05, 2016	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value		$ 1,566.9				$ 1,566.9		$ 2,081.1
Short-term investments, at amortized cost (which approximates fair value)		71.6				71.6		174.9
Common equity securities, at fair value		827.9				827.9		285.6
Other long-term investments		226.5				226.5		172.8
Total investments		2,692.9				2,692.9		2,714.4
Cash		53.3				53.3		80.2
Insurance premiums receivable		2.8				2.8		1.6
Deferred acquisition costs		13.0				13.0		10.6
Accounts receivable on unsettled investment sales		199.5				199.5		4.8
Goodwill and other intangible assets		49.5	$ 52.1		$ 51.5	49.5	$ 51.5	54.7	$ 54.2	$ 53.0
Other assets		62.8				62.8		64.1
Total assets held for sale		3,696.4				3,696.4		3,599.5
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Unearned insurance premiums		109.9				109.9		82.9
Debt		10.6				10.6		12.7
Accrued incentive compensation		63.3				63.3		95.7
Accounts payable on unsettled investment purchases		114.6				114.6		0.0
Other liabilities		44.9				44.9		46.9
Total liabilities held for sale		2,678.8				2,678.8		2,569.3
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		2.2			3.3	5.2	6.8
Net investment income		14.7			6.1	27.5	8.6
Net realized and unrealized investment gains		33.7			3.4	70.0	16.3
Total revenues		85.4			49.2	178.9	108.0
Loss and loss adjustment expenses		0.0			2.3	1.1	4.6
Insurance acquisition expenses		0.9			1.4	2.2	3.1
Other underwriting expenses		0.1			0.1	0.2	0.2
General and administrative expenses		57.5			45.2	117.9	99.9
Interest expense		0.5			0.9	0.9	2.1
Total expenses		85.8			74.3	177.9	162.8
Net income from discontinued operations, net of tax		3.4			17.0	35.7	64.4
Net (loss) gain on sale of discontinued operations, net of tax		(0.6)			366.6	(1.6)	366.6
Net change in foreign currency translation and benefit plan assets and obligations		0.6			(0.2)	1.4	(0.1)
Total comprehensive income		16.3			449.2	51.5	499.5
Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Total investments										$ 686.2
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net realized and unrealized investment gains					0.4		3.7
Net (loss) gain on sale of discontinued operations, net of tax		(0.6)		$ 4.0				363.2
Tranzact
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income tax benefit (expense)	$ 30.2							30.2
Net (loss) gain on sale of discontinued operations, net of tax	$ 51.9		$ (1.0)					51.9
Discontinued Operations
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Accounts receivable on unsettled investment sales		5.8				5.8		1.4
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Accounts payable on unsettled investment purchases		9.3				9.3		0.0
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		277.4			309.1	539.2	790.1
Net investment income		14.5			14.3	26.7	40.9
Net realized and unrealized investment gains		12.3			32.0	27.3	39.8
Other revenue		2.1			52.5	5.5	107.1
Total revenues		306.3			407.9	598.7	977.9
Loss and loss adjustment expenses		188.6			220.9	339.2	493.4
Insurance acquisition expenses		48.4			59.3	93.7	158.7
Other underwriting expenses		59.6			55.1	111.3	137.1
General and administrative expenses		8.8			52.9	13.8	118.3
Interest expense		3.3			5.8	6.6	17.1
Total expenses		308.7			394.0	564.6	924.6
Pre-tax (loss) income		(2.4)			13.9	34.1	53.3
Income tax benefit (expense)		5.8			3.1	1.6	11.1
Net income from discontinued operations, net of tax		3.4			17.0	35.7	64.4
Total (loss) income from discontinued operations		2.8			383.6	34.1	431.0
Net change in foreign currency translation and benefit plan assets and obligations		0.2			108.3	0.3	145.5
Total comprehensive income		3.0			491.9	34.4	576.5
Discontinued Operations | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) gain on sale of discontinued operations, net of tax		(0.6)			366.6	(1.6)	366.6
Discontinued Operations | Discontinued Operations, Held-for-sale
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value		2,288.6				2,288.6		2,175.7
Short-term investments, at amortized cost (which approximates fair value)		55.5				55.5		112.3
Common equity securities, at fair value		205.5				205.5		188.7
Other long-term investments		134.1				134.1		150.5
Total investments		2,683.7				2,683.7		2,627.2
Cash		71.3				71.3		70.5
Reinsurance recoverable on unpaid and paid losses		198.0				198.0		179.8
Insurance premiums receivable		245.4				245.4		229.8
Deferred acquisition costs		106.9				106.9		96.3
Deferred tax asset		130.1				130.1		126.7
Ceded unearned insurance and reinsurance premiums		56.6				56.6		44.2
Goodwill and other intangible assets		0.6				0.6		1.2
Other assets		198.0				198.0		222.4
Total assets held for sale		3,696.4				3,696.4		3,599.5
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Loss and loss adjustment expense reserves		1,411.2				1,411.2		1,370.6
Unearned insurance premiums		595.2				595.2		576.3
Debt		273.3				273.3		273.2
Accrued incentive compensation		39.5				39.5		44.3
Funds held under reinsurance treaties		210.2				210.2		153.0
Other liabilities		140.1				140.1		151.9
Total liabilities held for sale		2,678.8				2,678.8		2,569.3
Discontinued Operations | Sirius Group
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Net Assets		1,017.6				1,017.6		1,030.2
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		0.0			37.7		240.1
Net investment income		0.0			2.2		14.4
Net realized and unrealized investment gains		0.0			7.3		(1.5)
Other revenue		0.0			4.7		0.6
Total revenues		0.0			51.9		253.6
Loss and loss adjustment expenses		0.0			41.2		154.9
Insurance acquisition expenses		0.0			10.6		59.0
Other underwriting expenses		0.0			4.2		30.9
General and administrative expenses		0.0			2.3		10.4
Interest expense		0.0			1.3		7.9
Total expenses		0.0			59.6		263.1
Pre-tax (loss) income		0.0			(7.7)		(9.5)
Income tax benefit (expense)		0.0			2.2		3.1
Net income from discontinued operations, net of tax		0.0			(5.5)		(6.4)
Net (loss) gain on sale of discontinued operations, net of tax		(0.6)
Total (loss) income from discontinued operations		(0.6)			361.1	(0.6)	360.2
Net change in foreign currency translation and benefit plan assets and obligations					(5.2)		32.0
Total comprehensive income		(0.6)			469.2	(0.6)	505.5
Discontinued Operations | Sirius Group | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) gain on sale of discontinued operations, net of tax		(0.6)			366.6	(0.6)	366.6
Discontinued Operations | OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		277.4			271.4	539.2	550.0
Net investment income		14.5			12.1	26.7	26.5
Net realized and unrealized investment gains		12.3			24.7	27.3	41.3
Other revenue		2.1			0.8	5.5	1.7
Total revenues		306.3			309.0	598.7	619.5
Loss and loss adjustment expenses		188.6			179.7	339.2	338.5
Insurance acquisition expenses		48.4			48.7	93.7	99.7
Other underwriting expenses		59.6			50.9	111.3	106.2
General and administrative expenses		8.8			3.5	13.8	7.4
Interest expense		3.3			3.2	6.6	6.5
Total expenses		308.7			286.0	564.6	558.3
Pre-tax (loss) income		(2.4)			23.0	34.1	61.2
Income tax benefit (expense)		5.8			2.0	1.6	10.1
Net income from discontinued operations, net of tax		3.4			25.0	35.7	71.3
Total (loss) income from discontinued operations		3.4			25.0	35.7	71.3
Net change in foreign currency translation and benefit plan assets and obligations		0.2			0.2	0.3	0.2
Total comprehensive income		3.6			25.2	36.0	71.5
Discontinued Operations | Other Discontinued Operations
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					47.0		104.8
Total revenues					47.0		104.8
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					47.1		100.5
Interest expense					1.3		2.7
Total expenses					48.4		103.2
Pre-tax (loss) income					(1.4)		1.6
Income tax benefit (expense)					(1.1)		(2.1)
Net income from discontinued operations, net of tax					(2.5)		(0.5)
Total (loss) income from discontinued operations					(2.5)		(0.5)
Total comprehensive income					(2.5)		(0.5)
Discontinued Operations | Total
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net change in foreign currency translation and benefit plan assets and obligations		$ 0.2			(5.0)	0.3	32.2
Discontinued Operations | Tranzact
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income tax benefit (expense)								$ 8.8
Net income from discontinued operations, net of tax					(2.5)		(0.5)
Total (loss) income from discontinued operations						(1.0)
Total comprehensive income						(1.0)
Discontinued Operations | Tranzact | Net gain (loss) from sale of discontinued operations
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) gain on sale of discontinued operations, net of tax						$ (1.0)
Discontinued Operations | Sale of Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net change in foreign currency translation and benefit plan assets and obligations					$ 113.3		$ 113.3