Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value	$ 4,362.8				$ 4,362.8		$ 2,630.2
Short-term investments, at amortized cost (which approximates fair value)	421.8				421.8		211.2
Common equity securities, at fair value	407.5				407.5		1,113.9
Other long-term investments	329.2				329.2		315.8
Total investments	5,521.3				5,521.3		4,271.1
Cash	135.0				135.0		173.0
Reinsurance recoverable on unpaid losses	162.8				162.8		186.0
Reinsurance recoverable on paid losses	20.7				20.7		7.5
Insurance premiums receivable	240.4				240.4		220.3
Deferred acquisition costs	113.9				113.9		107.6
Deferred tax asset	122.2				122.2		112.8
Ceded unearned insurance premiums	34.5				34.5		29.5
Accounts receivable on unsettled investment sales	21.5				21.5		41.9
Goodwill and intangible assets	53.3				53.3		55.4
Other assets	239.4				239.4		273.2
Assets held for sale	384.6				384.6		4,790.4
Loss and loss adjustment expense reserves	1,376.6	$ 1,343.8		$ 1,350.8	1,376.6	$ 1,350.8	1,389.8	$ 1,304.0	$ 1,342.2
Unearned insurance premiums	619.7				619.7		610.5
Debt	289.8				289.8		337.6
Accrued incentive compensation	106.3				106.3		140.7
Ceded reinsurance payable	23.9				23.9		29.9
Funds held under insurance contracts	140.9				140.9		137.8
Accounts payable on unsettled investment purchases	69.9				69.9		0.0
Other liabilities	188.5				188.5		199.9
Total liabilities held for sale	150.6				150.6		3,068.4
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	274.7			321.8	556.8	611.1
Net investment income	19.2			13.2	37.1	25.8
Net realized and unrealized investment gains (losses)	28.1			(15.0)	57.6	10.0
Other revenue	36.2			48.7	76.0	76.1
Total revenues	358.2			368.7	727.5	723.0
Loss and loss adjustment expenses	182.0			196.2	343.1	364.1
Insurance and Reinsurance Acquisition Expenses	50.1			57.8	102.8	110.8
Other Underwriting Expense	51.0			53.0	106.4	109.0
General and administrative expenses	73.1			63.4	160.2	134.3
Interest expense	4.1			3.4	8.6	6.9
Total expenses	360.3			373.8	721.1	725.1
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax					(6.9)	62.9
Net gain on sale of discontinued operations	366.6			(0.1)	366.6	7.9
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash balances at beginning of period		167.2	$ 173.8		167.2	232.9
Cash balances at end of period	135.0			173.8	135.0	173.8
Net income tax refund from national governments					(13.5)	(7.5)
Net change in foreign currency translation	0.0			0.2	0.1	0.2
Total comprehensive income (loss)	449.2			(13.3)	499.5	12.3
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)	0.4			2.4	3.7	2.7
Net gain on sale of discontinued operations					366.6
Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	37.7			215.5	240.1	420.5
Net investment income	2.2			9.1	14.4	18.1
Net realized and unrealized investment gains (losses)	7.3			(46.1)	(1.5)	32.3
Other revenue	51.7			40.1	105.4	50.1
Total revenues	98.9			218.6	358.4	521.0
Loss and loss adjustment expenses	41.2			92.1	154.9	189.6
Insurance and Reinsurance Acquisition Expenses	10.6			47.5	59.0	90.1
Other Underwriting Expense	4.2			27.7	30.9	53.2
General and administrative expenses	49.4			44.7	110.9	90.2
Interest expense	2.6			7.5	10.6	15.1
Total expenses	108.0			219.5	366.3	438.2
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(9.1)			(0.9)	(7.9)	82.8
Discontinued Operation, Tax Effect of Discontinued Operation	1.1			0.5	1.0	(19.9)
Net gain on sale of discontinued operations	(8.0)			(0.4)	(6.9)	62.9
Net loss from discontinued operations, net of tax	358.6			(0.5)	359.7	70.8
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash used for operations - discontinued operations					(43.1)	(12.9)
Net cash provided from investing activities - discontinued operations					229.6	59.3
Net cash provided from (used for) financing activities - discontinued operations					9.0	(2.0)
Effect of exchange rate changes on cash					0.0	(2.1)
Net change in cash during the period					195.5	42.3
Cash balances at beginning of period		$ 148.9	156.8		148.9	114.5
Cash balances at end of period	1.3			156.8	1.3	156.8
Interest paid					(1.4)	(14.5)
Net income tax refund from national governments					(31.8)	(24.6)
Net change in foreign currency translation	(5.2)			44.0		(43.6)
Total comprehensive income (loss)	466.7			43.5	505.0	27.2
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	366.6				366.6
Discontinued Operations [Member] | Discontinued Operations, Held-for-sale [Member]
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value	14.4				14.4		2,383.5
Short-term investments, at amortized cost (which approximates fair value)	4.2				4.2		352.1
Common equity securities, at fair value	0.0				0.0		174.4
Other long-term investments	0.0				0.0		72.2
Total investments	18.6				18.6		2,982.2
Cash	1.9				1.9		150.2
Reinsurance recoverable on unpaid losses	0.4				0.4		283.6
Reinsurance recoverable on paid losses	0.0				0.0		10.2
Insurance premiums receivable	2.4				2.4		326.6
Deferred acquisition costs	0.0				0.0		74.6
Deferred tax asset	0.0				0.0		303.1
Ceded unearned insurance premiums	0.0				0.0		87.7
Goodwill and intangible assets	315.1				315.1		330.5
Other assets	46.2				46.2		212.7
Assets held for sale	384.6				384.6		4,790.4
Loss and loss adjustment expense reserves	5.7				5.7		1,650.4
Unearned insurance premiums	1.8				1.8		344.3
Debt	96.9				96.9		506.4
Deferred tax liability	8.7				8.7		270.6
Accrued incentive compensation	1.0				1.0		64.2
Ceded reinsurance payable	(0.2)				(0.2)		67.7
Funds held under insurance contracts	0.0				0.0		52.9
Other liabilities	36.7				36.7		111.9
Total liabilities held for sale	150.6				150.6		3,068.4
Discontinued Operations [Member] | Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Accounts receivable on unsettled investment sales	0.0				0.0		29.0
Accounts payable on unsettled investment purchases	0.0				0.0		0.0
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	37.7			215.5	240.1	420.5
Net investment income	2.2			9.1	14.4	18.1
Net realized and unrealized investment gains (losses)	7.3			(46.1)	(1.5)	32.3
Other revenue	4.7			5.4	0.6	(22.0)
Total revenues	51.9			183.9	253.6	448.9
Loss and loss adjustment expenses	41.2			92.1	154.9	189.6
Insurance and Reinsurance Acquisition Expenses	10.6			47.5	59.0	90.1
Other Underwriting Expense	4.2			27.7	30.9	53.2
General and administrative expenses	0.2			6.7	8.3	13.0
Interest expense	1.3			6.6	7.9	13.4
Total expenses	57.5			180.6	261.0	359.3
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(5.6)			3.3	(7.4)	89.6
Discontinued Operation, Tax Effect of Discontinued Operation	2.2			0.5	3.1	(19.9)
Net gain on sale of discontinued operations	(3.4)			3.8	(4.3)	69.7
Net loss from discontinued operations, net of tax	363.2			3.8	362.3	69.7
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net Assets	234.0				234.0		$ 1,722.0
Net change in foreign currency translation	(5.2)			44.0	32.0	(43.6)
Total comprehensive income (loss)	471.3			47.8	507.6	26.1
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	366.6				366.6
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	47.0			34.7	104.8	72.1
Total revenues	47.0			34.7	104.8	72.1
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	49.2			38.0	102.6	77.2
Interest expense	1.3			0.9	2.7	1.7
Total expenses	50.5			38.9	105.3	78.9
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(3.5)			(4.2)	(0.5)	(6.8)
Discontinued Operation, Tax Effect of Discontinued Operation	(1.1)			0.0	(2.1)	0.0
Net gain on sale of discontinued operations	(4.6)			(4.2)	(2.6)	(6.8)
Net loss from discontinued operations, net of tax	(4.6)			(4.3)	(2.6)	1.1
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Total comprehensive income (loss)	(4.6)			(4.3)	(2.6)	1.1
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			0.0	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		0.3
Net loss from discontinued operations, net of tax				(0.2)		(0.3)
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	0.0			0.3		0.3
Discontinued Operations [Member] | Esurance
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations			$ 10.3	(0.3)	$ 28.3	7.6
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			$ (0.4)		$ 7.6