Discontinued Operations - Financial Statement Disclosures (Details) - USD ($)			3 Months Ended				6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 28, 2017	Oct. 05, 2016	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Jun. 30, 2017	Dec. 31, 2015
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value			$ 1,490,000,000					$ 1,490,000,000		$ 2,081,100,000
Short-term investments, at amortized cost (which approximates fair value)			786,500,000					786,500,000		174,900,000
Common equity securities, at fair value			774,400,000					774,400,000		285,600,000
Other long-term investments			229,600,000					229,600,000		172,800,000
Total investments			3,280,500,000					3,280,500,000		2,714,400,000
Cash			48,600,000					48,600,000		80,200,000
Insurance premiums receivable			4,500,000					4,500,000		1,600,000
Deferred acquisition costs			14,100,000					14,100,000		10,600,000
Accounts receivable on unsettled investment sales			190,100,000					190,100,000		4,800,000
Goodwill and other intangible assets			37,800,000		$ 47,700,000	$ 41,600,000	$ 41,600,000	37,800,000	$ 47,700,000	45,200,000	$ 40,300,000	$ 45,500,000
Accrued investment income			15,400,000					15,400,000		14,800,000
Other assets			46,100,000					46,100,000		49,100,000
Total assets held for sale			0					0		3,599,500,000
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Unearned insurance premiums			118,500,000					118,500,000		82,900,000
Debt			9,400,000					9,400,000		12,700,000
Accrued incentive compensation			76,900,000					76,900,000		95,700,000
Accounts payable on unsettled investment purchases			49,700,000					49,700,000		0
Other liabilities			45,400,000					45,400,000		43,600,000
Total liabilities held for sale			0					0		2,569,300,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			2,400,000		3,400,000			7,600,000	10,200,000
Net investment income			12,200,000		9,600,000			39,700,000	18,200,000
Net realized and unrealized investment gains			32,500,000		10,900,000			102,500,000	27,200,000
Total revenues			87,500,000		56,800,000			259,800,000	163,200,000
Loss and loss adjustment expenses			0		2,200,000			1,100,000	6,800,000
Insurance acquisition expenses			900,000		1,300,000			3,100,000	4,400,000
Other underwriting expenses			100,000		200,000			300,000	400,000
General and administrative expenses			44,100,000		42,400,000			162,200,000	144,900,000
Interest expense			900,000		500,000			1,800,000	2,600,000
Total expenses			79,100,000		70,600,000			257,200,000	236,000,000
Net (loss) income from discontinued operations, net of tax			(15,200,000)		36,500,000			20,500,000	100,900,000
Gain from sale of other discontinued operations, net of tax			554,300,000		47,900,000			552,700,000	414,500,000
Total income (loss) from discontinued operations			(539,100,000)		(84,400,000)			(573,200,000)	(515,400,000)
Net change in foreign currency translation and benefit plan assets and obligations			0		200,000			200,000	(400,000)
Total comprehensive income			565,100,000		90,700,000			608,100,000	586,100,000
Discontinued Operations, Held-for-sale
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Goodwill and other intangible assets			0					0		1,200,000
Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Total investments												$ 686,200,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net realized and unrealized investment gains						$ 400,000	$ 3,700,000
Gain from sale of other discontinued operations, net of tax					(4,000,000)					363,200,000
OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net (loss) income from discontinued operations, net of tax			(15,200,000)		22,500,000			20,500,000	93,800,000
Gain from sale of other discontinued operations, net of tax	$ 554,600,000
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					14,800,000				119,600,000
Total revenues					14,800,000				119,600,000
Loss and loss adjustment expenses					0				0
Insurance acquisition expenses					0				0
Other underwriting expenses					0				0
General and administrative expenses					16,200,000				116,700,000
Interest expense					500,000				3,200,000
Total expenses					16,700,000				119,900,000
Pre-tax (loss) income					(1,900,000)				(300,000)
Income tax benefit (expense)		$ 30,200,000			15,900,000				13,800,000	30,200,000
Net (loss) income from discontinued operations, net of tax					14,000,000				13,500,000
Gain from sale of other discontinued operations, net of tax		$ 51,900,000		$ (1,000,000)						51,900,000
Total comprehensive income					65,900,000
Discontinued Operations
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Accounts receivable on unsettled investment sales										1,400,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Total income (loss) from discontinued operations			539,100,000		84,400,000			573,200,000	515,400,000
Net change in foreign currency translation and benefit plan assets and obligations									32,500,000
Discontinued Operations | Discontinued Operations, Held-for-sale
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value										2,175,700,000
Short-term investments, at amortized cost (which approximates fair value)										112,300,000
Common equity securities, at fair value										188,700,000
Other long-term investments										150,500,000
Total investments										2,627,200,000
Cash										70,500,000
Reinsurance recoverable on unpaid and paid losses										179,800,000
Insurance premiums receivable										229,800,000
Deferred acquisition costs										96,300,000
Deferred tax asset										126,700,000
Ceded unearned insurance and reinsurance premiums										44,200,000
Goodwill and other intangible assets										1,200,000
Accrued investment income										11,300,000
Other assets										211,100,000
Total assets held for sale										3,599,500,000
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Loss and loss adjustment expense reserves										1,370,600,000
Unearned insurance premiums										576,300,000
Debt										273,200,000
Accrued incentive compensation										44,300,000
Funds held under reinsurance treaties										153,000,000
Other liabilities										151,900,000
Total liabilities held for sale										2,569,300,000
Discontinued Operations | Sirius Group
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Net Assets										1,030,200,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			0		0				240,100,000
Net investment income			0		0				14,400,000
Net realized and unrealized investment gains			0		0				(1,500,000)
Other revenue			0		0				600,000
Total revenues			0		0				253,600,000
Loss and loss adjustment expenses			0		0				154,900,000
Insurance acquisition expenses			0		0				59,000,000
Other underwriting expenses			0		0				30,900,000
General and administrative expenses			0		0				10,400,000
Interest expense			0		0				7,900,000
Total expenses			0		0				263,100,000
Pre-tax (loss) income			0		0				(9,500,000)
Income tax benefit (expense)			0		0				3,100,000
Net (loss) income from discontinued operations, net of tax			0		0				(6,400,000)
Gain from sale of other discontinued operations, net of tax			(800,000)					1,000,000
Total income (loss) from discontinued operations			(200,000)		(4,000,000)			(800,000)	356,200,000
Net change in foreign currency translation and benefit plan assets and obligations					0				32,000,000
Total comprehensive income			(200,000)		(4,000,000)			(800,000)	501,500,000
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			(200,000)		(4,000,000)			(800,000)	362,600,000
Discontinued Operations | OneBeacon
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			268,400,000		277,900,000			807,600,000	827,900,000
Net investment income			13,000,000		11,800,000			39,700,000	38,300,000
Net realized and unrealized investment gains			11,500,000		15,500,000			38,800,000	56,800,000
Other revenue			2,200,000		1,800,000			7,700,000	3,500,000
Total revenues			295,100,000		307,000,000			893,800,000	926,500,000
Loss and loss adjustment expenses			206,800,000		162,800,000			546,000,000	501,300,000
Insurance acquisition expenses			51,900,000		55,100,000			145,600,000	154,800,000
Other underwriting expenses			44,900,000		49,400,000			156,200,000	155,600,000
General and administrative expenses			7,400,000		3,500,000			21,200,000	10,900,000
Interest expense			3,400,000		3,300,000			10,000,000	9,800,000
Total expenses			314,400,000		274,100,000			879,000,000	832,400,000
Pre-tax (loss) income			(19,300,000)		32,900,000			14,800,000	94,100,000
Income tax benefit (expense)			4,100,000		(10,400,000)			5,700,000	(300,000)
Net (loss) income from discontinued operations, net of tax			(15,200,000)		22,500,000			20,500,000	93,800,000
Total income (loss) from discontinued operations			539,300,000		22,500,000			575,000,000	93,800,000
Net change in foreign currency translation and benefit plan assets and obligations			100,000		(300,000)			300,000	500,000
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax			2,900,000					2,900,000
Total comprehensive income			542,300,000		22,200,000			578,200,000	94,300,000
Discontinued Operations | OneBeacon | 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			554,500,000					554,500,000
Discontinued Operations | Total
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums			268,400,000		277,900,000			807,600,000	1,068,000,000
Net investment income			13,000,000		11,800,000			39,700,000	52,700,000
Net realized and unrealized investment gains			11,500,000		15,500,000			38,800,000	55,300,000
Other revenue			2,200,000		16,600,000			7,700,000	123,700,000
Total revenues			295,100,000		321,800,000			893,800,000	1,299,700,000
Loss and loss adjustment expenses			206,800,000		162,800,000			546,000,000	656,200,000
Insurance acquisition expenses			51,900,000		55,100,000			145,600,000	213,800,000
Other underwriting expenses			44,900,000		49,400,000			156,200,000	186,500,000
General and administrative expenses			7,400,000		19,700,000			21,200,000	138,000,000
Interest expense			3,400,000		3,800,000			10,000,000	20,900,000
Total expenses			314,400,000		290,800,000			879,000,000	1,215,400,000
Pre-tax (loss) income			(19,300,000)		31,000,000			14,800,000	84,300,000
Income tax benefit (expense)			4,100,000		5,500,000			5,700,000	16,600,000
Net (loss) income from discontinued operations, net of tax			(15,200,000)		36,500,000			20,500,000	100,900,000
Total income (loss) from discontinued operations			539,100,000		84,400,000			573,200,000	515,400,000
Net change in foreign currency translation and benefit plan assets and obligations			100,000		(300,000)			300,000	113,300,000
Recognition of benefit plan assets and obligations from the sale of OneBeacon, net of tax			2,900,000					2,900,000
Total comprehensive income			542,100,000		84,100,000			576,400,000	661,200,000
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			$ 554,300,000		47,900,000			552,700,000	414,500,000
Discontinued Operations | Tranzact
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income tax benefit (expense)										$ 30.2
Net (loss) income from discontinued operations, net of tax					2,100,000				2,600,000
Total income (loss) from discontinued operations					65,900,000			(1,000,000)	65,400,000
Total comprehensive income								(1,000,000)	65,400,000
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					51,900,000			$ (1,000,000)	51,900,000
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					$ 0				$ 113,300,000