Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		60 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value	$ 4,131.5			$ 4,131.5		$ 4,131.5	$ 2,630.2
Short-term investments, at amortized cost (which approximates fair value)	305.6			305.6		305.6	211.2
Common equity securities, at fair value	425.0			425.0		425.0	1,113.9
Other long-term investments	351.4			351.4		351.4	315.8
Total investments	5,213.5			5,213.5		5,213.5	4,271.1
Cash	244.7			244.7		244.7	173.0
Reinsurance recoverable on unpaid losses	169.4			169.4		169.4	186.0
Reinsurance recoverable on paid losses	16.4			16.4		16.4	7.5
Insurance premiums receivable	264.6			264.6		264.6	220.3
Deferred acquisition costs	109.6			109.6		109.6	107.6
Deferred tax asset	119.5			119.5		119.5	112.8
Ceded unearned insurance premiums	35.1			35.1		35.1	29.5
Accounts receivable on unsettled investment sales	179.0			179.0		179.0	41.9
Goodwill and other intangible assets	59.3	$ 53.3	$ 58.0	59.3	$ 58.0	59.3	55.4	$ 60.5	$ 62.5
Other assets	226.3			226.3		226.3	273.2
Assets held for sale	27.0			27.0		27.0	4,790.4
Loss and loss adjustment expense reserves	1,362.0	1,376.6	1,427.4	1,362.0	1,427.4	1,362.0	1,389.8	$ 1,350.8	$ 1,342.2
Unearned insurance premiums	674.1			674.1		674.1	610.5
Debt	288.5			288.5		288.5	337.6
Accrued incentive compensation	124.2			124.2		124.2	140.7
Ceded reinsurance payable	28.2			28.2		28.2	29.9
Funds held under insurance contracts	156.2			156.2		156.2	137.8
Accounts payable on unsettled investment purchases	30.0			30.0		30.0	0.0
Other liabilities	210.9			210.9		210.9	220.9
Total liabilities held for sale	7.8			7.8		7.8	3,047.4
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	281.3		284.9	838.1	896.0
Net investment income	22.5		16.8	59.6	42.6
Net realized and unrealized investment gains (losses)	26.4		(43.9)	84.0	(33.9)
Other revenue	34.8		38.3	110.8	114.4
Total revenues	365.0		296.1	1,092.5	1,019.1
Loss and loss adjustment expenses	165.0		170.0	508.1	534.1
Insurance and Reinsurance Acquisition Expenses	56.4		55.3	159.2	166.1
Other Underwriting Expense	49.6		56.5	156.0	165.5
General and administrative expenses	70.2		97.3	230.4	231.6
Interest expense	3.8		3.9	12.4	10.8
Total expenses	345.0		383.0	1,066.1	1,108.1
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax				7.1	59.1
Net gain on sale of discontinued operations				414.5	18.2
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period				167.2	232.9
Cash balances at end of period	244.7		173.0	244.7	173.0	244.7
Net income tax refund from national governments				(13.5)	(8.0)
Net change in foreign currency translation	0.2		0.1	0.3	0.3
Total comprehensive income (loss)	91.9		(73.8)	591.4	(61.5)
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 (losses)			0.8	3.7	3.5
Net gain on sale of discontinued operations	(4.0)	366.6	0.0	362.6	0.0
Tranzact [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Discontinued Operation, Tax Effect of Discontinued Operation	30.2
Net gain on sale of discontinued operations	51.9
Esurance
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations			10.3		18.2
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash Reclassified to Assets Held-for-sale				2.6
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations [Member]
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash sold as part of sale of subsidiaries	345.8			345.8		345.8
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash Reclassified to Assets Held-for-sale	345.8			345.8		345.8
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	0.0		202.8	240.1	623.3
Net investment income	0.0		10.5	14.4	28.6
Net realized and unrealized investment gains (losses)	0.0		(0.9)	(1.5)	31.4
Other revenue	14.8		40.3	120.2	90.4
Total revenues	14.8		252.7	373.2	773.7
Loss and loss adjustment expenses	0.0		121.7	154.9	311.3
Insurance and Reinsurance Acquisition Expenses	0.0		45.4	59.0	135.5
Other Underwriting Expense	0.0		24.8	30.9	78.0
General and administrative expenses	16.2		54.7	127.1	144.9
Interest expense	0.5		7.6	11.1	22.7
Total expenses	16.7		254.2	383.0	692.4
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(1.9)		(1.5)	(9.8)	81.3
Discontinued Operation, Tax Effect of Discontinued Operation	15.9		(2.3)	16.9	(22.2)
Net gain on sale of discontinued operations	14.0		(3.8)	7.1	59.1
Net loss from discontinued operations, net of tax	61.9		6.5	421.6	77.3
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash (used for) provided from operations - discontinued operations				(16.2)	22.7
Net cash provided from investing activities - discontinued operations				213.3	2.8
Net cash (used for) provided from financing activities - discontinued operations				(0.2)	19.6
Effect of exchange rate changes on cash				0.0	(4.1)
Net change in cash during the period				196.9	41.0
Cash balances at beginning of period				150.1	116.6
Cash balances at end of period	0.0		157.6	0.0	157.6	0.0
Interest paid				(1.4)	(28.1)
Net income tax refund from national governments				(31.8)	(30.3)
Net change in foreign currency translation	0.0		(18.5)		(62.1)
Total comprehensive income (loss)	61.9		(12.0)	566.9	15.2
Discontinued Operations [Member] | Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	(4.0)			362.6
Discontinued Operations [Member] | Discontinued Operations, Held-for-sale [Member]
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value	13.4			13.4		13.4	2,383.5
Short-term investments, at amortized cost (which approximates fair value)	1.3			1.3		1.3	352.1
Common equity securities, at fair value	0.0			0.0		0.0	174.4
Other long-term investments	0.0			0.0		0.0	72.2
Total investments	14.7			14.7		14.7	2,982.2
Cash	3.8			3.8		3.8	150.1
Reinsurance recoverable on unpaid losses	0.6			0.6		0.6	283.6
Reinsurance recoverable on paid losses	0.0			0.0		0.0	10.2
Insurance premiums receivable	2.0			2.0		2.0	326.6
Deferred acquisition costs	0.0			0.0		0.0	74.6
Deferred tax asset	0.0			0.0		0.0	303.1
Ceded unearned insurance premiums	0.0			0.0		0.0	87.7
Goodwill and other intangible assets	5.2			5.2		5.2	330.5
Other assets	0.7			0.7		0.7	212.8
Assets held for sale	27.0			27.0		27.0	4,790.4
Loss and loss adjustment expense reserves	6.0			6.0		6.0	1,650.4
Unearned insurance premiums	1.6			1.6		1.6	344.3
Debt	0.0			0.0		0.0	506.4
Deferred tax liability	0.0			0.0		0.0	270.6
Accrued incentive compensation	0.0			0.0		0.0	64.2
Ceded reinsurance payable	(0.5)			(0.5)		(0.5)	67.7
Funds held under insurance contracts	0.0			0.0		0.0	52.9
Other liabilities	0.7			0.7		0.7	90.9
Total liabilities held for sale	7.8			7.8		7.8	3,047.4
Discontinued Operations [Member] | Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Accounts receivable on unsettled investment sales	0.0			0.0		0.0	29.0
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums		0.0	202.8	240.1	623.3
Net investment income		0.0	10.5	14.4	28.6
Net realized and unrealized investment gains (losses)		0.0	(0.9)	(1.5)	31.4
Other revenue		0.0	1.8	0.6	(20.2)
Total revenues		0.0	214.2	253.6	663.1
Loss and loss adjustment expenses		0.0	121.7	154.9	311.3
Insurance and Reinsurance Acquisition Expenses		0.0	45.4	59.0	135.5
Other Underwriting Expense		0.0	24.8	30.9	78.0
General and administrative expenses		0.0	9.4	10.4	22.4
Interest expense		0.0	6.6	7.9	20.0
Total expenses		0.0	207.9	263.1	567.2
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax		0.0	6.3	(9.5)	95.9
Discontinued Operation, Tax Effect of Discontinued Operation		0.0	(2.3)	3.1	(22.2)
Net gain on sale of discontinued operations		0.0	4.0	(6.4)	73.7
Net loss from discontinued operations, net of tax		(4.0)	4.0	356.2	73.7
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net Assets	19.2			19.2		19.2	$ 1,743.0
Net change in foreign currency translation		0.0	(18.5)	32.0	(62.1)
Total comprehensive income (loss)		(4.0)	(14.5)	501.5	11.6
Discontinued Operations [Member] | Sirius Group | Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations		$ (4.0)		362.6
Discontinued Operations [Member] | Tranzact [Member]
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Other liabilities	2.8			2.8		2.8
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Discontinued Operation, Tax Effect of Discontinued Operation	(16.1)
Discontinued Operations [Member] | Tranzact [Member] | Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations				51.9
Discontinued Operations [Member] | Other Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	0.0		0.0	0.0	0.0
Net investment income	0.0		0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	0.0		0.0	0.0	0.0
Other revenue	14.8		38.5	119.6	110.6
Total revenues	14.8		38.5	119.6	110.6
Loss and loss adjustment expenses	0.0		0.0	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.0		0.0	0.0	0.0
Other Underwriting Expense	0.0		0.0	0.0	0.0
General and administrative expenses	16.2		45.3	116.7	122.5
Interest expense	0.5		1.0	3.2	2.7
Total expenses	16.7		46.3	119.9	125.2
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(1.9)		(7.8)	(0.3)	(14.6)
Discontinued Operation, Tax Effect of Discontinued Operation	15.9		0.0	13.8	0.0
Net gain on sale of discontinued operations	14.0		(7.8)	13.5	(14.6)
Net loss from discontinued operations, net of tax	65.9		2.5	65.4	3.6
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Total comprehensive income (loss)	65.9		2.5	65.4	3.6
Discontinued Operations [Member] | Sale of Sirius Group [Member]
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net change in foreign currency translation				113.3
Discontinued Operations [Member] | OneBeacon Runoff [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations					0.3
Net loss from discontinued operations, net of tax			(0.1)		(0.4)
Discontinued Operations [Member] | OneBeacon Runoff [Member] | Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	51.9		0.0		0.3
Discontinued Operations [Member] | Esurance
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	10.3		10.3		17.9	28.3
Discontinued Operations [Member] | Esurance | Net gain (loss) from sale of discontinued operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	0.0		10.3		17.9
Discontinued Operations, Held-for-sale [Member]
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period				150.1	116.6
Cash balances at end of period	$ 3.8		$ 157.6	$ 3.8	$ 157.6	$ 3.8