Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments	$ 2,602.2			$ 2,602.2			$ 2,422.0
Short-term investments	325.0			325.0			376.8
Common equity securities	482.1			482.1			611.7
Convertible fixed maturity and preferred investments	5.6			5.6			13.9
Other long-term investments	294.6			294.6			318.0
Total investments	3,709.5			3,709.5			3,742.4
Cash	200.0			200.0			261.7
Reinsurance recoverable on unpaid losses	214.6			214.6			161.7
Reinsurance recoverable on paid losses	22.6			22.6			12.2
Insurance premiums receivable	273.3			273.3			241.1
Funds held by ceding entities	4.6			4.6			37.1
Deferred acquisition costs	110.8			110.8			107.2
Deferred tax asset	133.9			133.9			114.6
Ceded unearned insurance premiums	41.4			41.4			17.8
Goodwill and intangible assets	381.7			381.7			351.2
Other assets	307.7			307.7			311.6
Assets held for sale	4,528.1			4,528.1			4,630.6
Loss and loss adjustment expense reserves	1,433.3	$ 1,169.5	$ 1,137.8	1,433.3	$ 1,169.5	$ 1,357.0	1,350.0	$ 1,054.3
Unearned insurance premiums	671.3			671.3			616.7
Debt	383.3			383.3			343.1
Accrued incentive compensation	121.0			121.0			108.1
Ceded reinsurance payable	46.8			46.8			34.2
Funds held under insurance contracts	100.3			100.3			81.0
Other liabilities	304.9			304.9			278.4
Total liabilities held for sale	3,032.8			3,032.8			3,105.3
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	284.9	307.2		896.0	879.1
Net investment income	16.8	13.8		42.6	44.7
Net realized and unrealized investment (losses) gains	(43.9)	(27.5)		(33.9)	64.4
Other revenue	76.9	29.7		225.4	60.3
Total revenues	334.7	323.2		1,130.1	1,048.5
Loss and loss adjustment expenses	170.0	185.5		534.1	527.3
Insurance and Reinsurance Acquisition Expenses	55.3	55.2		166.1	153.0
Other Underwriting Expense	56.5	51.3		165.5	152.6
General and administrative expenses	142.6	63.0		354.1	168.3
Interest expense	4.9	3.7		13.5	10.6
Total expenses	429.3	358.7		1,233.3	1,011.8
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax				73.3	166.8
Net gain on sale of discontinued operations	10.3	7.0		18.2	9.7
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	0.0	(0.2)		0.3	(0.1)
Comprehensive (loss) income	(73.8)	(22.9)		(61.5)	201.8
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net change in cash during the period				17.5	(27.6)
Cash balances at beginning of period				238.0	233.5
Cash balances at end of period	183.2	395.4		183.2	395.4
Net income tax refund from national governments				(8.0)	(0.3)
Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Total investments	510.8			510.8
Fireman's Fund Insurance Company [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations		14.0
OneBeacon Runoff [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations		6.7	5.3
Esurance
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations					3.2
Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	202.8	231.7		623.3	656.6
Net investment income	10.5	9.7		28.6	30.0
Net realized and unrealized investment (losses) gains	(0.9)	48.3		31.4	134.1
Other revenue	1.7	0.0		(20.6)	(47.3)
Total revenues	214.2	266.9		662.7	773.4
Loss and loss adjustment expenses	121.7	103.6		311.3	273.1
Insurance and Reinsurance Acquisition Expenses	45.4	50.2		135.5	146.4
Other Underwriting Expense	24.8	30.5		78.0	93.9
General and administrative expenses	9.4	6.3		22.4	21.6
Interest expense	6.6	6.6		20.0	19.8
Total expenses	207.9	197.2		567.2	554.8
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	6.2	69.7		95.5	218.6
Discontinued Operation, Tax Effect of Discontinued Operation	(2.3)	(12.8)		(22.2)	(51.8)
Net gain on sale of discontinued operations	3.9	56.9	$ 2.7	73.3	166.8
Net loss from discontinued operations, net of tax	14.2	63.9		91.5	176.5
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	(18.4)	(64.1)		(62.1)	(99.9)
Comprehensive (loss) income	(4.2)	(0.2)		29.4	76.6
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash provided from operations - discontinued operations				6.7	49.8
Net cash provided from investing activities - discontinued operations				35.7	47.5
Net cash used for financing activities - discontinued operations				(2.4)	(55.6)
Effect of exchange rate changes on cash				(4.1)	(10.6)
Net change in cash during the period				35.9	31.1
Cash balances at beginning of period				111.5	93.2
Cash balances at end of period	147.4	124.3		147.4	124.3
Interest paid				(25.5)	(34.9)
Net income tax refund from national governments				(30.3)	7.5
Discontinued Operations [Member] | Discontinued Operations, Held-for-sale [Member]
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments	2,394.3			2,394.3			2,362.3
Short-term investments	360.5			360.5			494.9
Common equity securities	164.3			164.3			189.9
Convertible fixed maturity and preferred investments	0.0			0.0			6.6
Other long-term investments	82.1			82.1			89.0
Total investments	3,001.2			3,001.2			3,142.7
Cash	147.4			147.4			111.5
Reinsurance recoverable on unpaid losses	276.5			276.5			322.2
Reinsurance recoverable on paid losses	11.0			11.0			11.4
Insurance premiums receivable	416.6			416.6			306.6
Funds held by ceding entities	81.1			81.1			91.9
Deferred acquisition costs	82.7			82.7			69.9
Deferred tax asset	309.6			309.6			341.5
Ceded unearned insurance premiums	118.4			118.4			76.2
Goodwill and intangible assets	10.2			10.2			15.2
Other assets	73.4			73.4			83.4
Assets held for sale	4,528.1			4,528.1			4,630.6
Loss and loss adjustment expense reserves	1,662.9			1,662.9			1,809.8
Unearned insurance premiums	429.4			429.4			338.6
Debt	403.5			403.5			403.5
Deferred tax liability	257.8			257.8			282.8
Accrued incentive compensation	55.5			55.5			76.5
Ceded reinsurance payable	131.4			131.4			71.5
Funds held under insurance contracts	45.1			45.1			57.9
Other liabilities	47.2			47.2			64.7
Total liabilities held for sale	3,032.8			3,032.8			3,105.3
Discontinued Operations [Member] | Sirius Group
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	202.8	231.4		623.3	656.5
Net investment income	10.5	9.7		28.6	30.0
Net realized and unrealized investment (losses) gains	(0.9)	48.3		31.4	134.1
Other revenue	1.8	(22.8)		(20.2)	(47.3)
Total revenues	214.2	266.6		663.1	773.3
Loss and loss adjustment expenses	121.7	103.6		311.3	273.8
Insurance and Reinsurance Acquisition Expenses	45.4	50.1		135.5	146.3
Other Underwriting Expense	24.8	29.7		78.0	91.7
General and administrative expenses	9.4	6.3		22.4	21.6
Interest expense	6.6	6.6		20.0	19.8
Total expenses	207.9	196.3		567.2	553.2
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	6.3	70.3		95.9	220.1
Discontinued Operation, Tax Effect of Discontinued Operation	(2.3)	(13.1)		(22.2)	(52.4)
Net gain on sale of discontinued operations	4.0	57.2		73.7	167.7
Net loss from discontinued operations, net of tax	4.0	57.2		73.7	167.7
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	(18.4)	(64.1)		(62.1)	(99.9)
Comprehensive (loss) income	(14.4)	(6.9)		11.6	67.8
Discontinued Operations [Member] | Other Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	0.0	0.3		0.0	0.1
Net investment income	0.0	0.0		0.0	0.0
Net realized and unrealized investment (losses) gains	0.0	0.0		0.0	0.0
Other revenue	(0.1)	0.0		(0.4)	0.0
Total revenues	(0.1)	0.3		(0.4)	0.1
Loss and loss adjustment expenses	0.0	0.0		0.0	(0.7)
Insurance and Reinsurance Acquisition Expenses	0.0	0.1		0.0	0.1
Other Underwriting Expense	0.0	0.8		0.0	2.2
General and administrative expenses	0.0	0.0		0.0	0.0
Interest expense	0.0	0.0		0.0	0.0
Total expenses	0.0	0.9		0.0	1.6
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(0.1)	(0.6)		(0.4)	(1.5)
Discontinued Operation, Tax Effect of Discontinued Operation	0.0	0.3		0.0	0.6
Net gain on sale of discontinued operations	(0.1)	(0.3)		(0.4)	(0.9)
Net loss from discontinued operations, net of tax	10.2	6.7		17.8	8.8
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	0.0	0.0		0.0	0.0
Comprehensive (loss) income	10.2	6.7		17.8	8.8
Discontinued Operations [Member] | OneBeacon | Discontinued Operations, Held-for-sale [Member]
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Other assets	0.0			0.0			$ 58.1
Discontinued Operations [Member] | Fireman's Fund Insurance Company [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	0.0	14.0		0.0	14.0
Discontinued Operations [Member] | OneBeacon Runoff [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	0.0	(7.0)		0.3	(7.5)
Discontinued Operations [Member] | Esurance
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	$ 10.3	$ 0.0		$ 17.9	$ 3.2