Discontinued Operations - Financial Statement Disclosures (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended	54 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Jun. 30, 2014	Sep. 30, 2015	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value	$ 2,613.2				$ 2,613.2	$ 2,639.7
Short-term	261.7				261.7	211.3
Common equities	443.0				443.0	1,113.9
Other	321.8				321.8	315.8
Total investments	3,639.7				3,639.7	4,280.7
Cash	178.8				178.8	179.3
Reinsurance recoverable on unpaid losses	150.9				150.9	186.5
Reinsurance recoverable on paid losses	16.6				16.6	7.5
Insurance premiums receivable	234.3				234.3	223.3
Deferred acquisition costs	109.8				109.8	107.6
Deferred tax asset	133.6				133.6	112.8
Ceded unearned insurance premiums	36.3				36.3	29.5
Accounts receivable on unsettled investment sales	4.5				4.5	41.9
Goodwill and intangible assets	371.5				371.5	375.7
Other assets	286.9				286.9	316.8
Assets held for sale	5,215.5				5,215.5	4,407.0
Loss and loss adjustment expense reserves	1,349.6	$ 1,310.6			1,349.6	1,395.8	$ 1,350.0
Unearned insurance premiums	626.5				626.5	612.6
Debt	532.6				532.6	440.5
Accrued incentive compensation	114.2				114.2	141.7
Ceded reinsurance payable	31.8				31.8	30.5
Funds held under insurance contracts	134.9				134.9	137.8
Accounts payable on unsettled investment purchases	6.9				6.9	0.0
Other liabilities	229.8				229.8	264.7
Total liabilities held for sale	3,121.1				3,121.1	2,884.0
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	282.1	289.3
Net investment income	17.9	12.6
Net realized and unrealized investment gains	29.5	25.0
Other revenue	97.6	64.9
Total revenues	427.1	391.8
Loss and loss adjustment expenses	161.1	167.9
Insurance and Reinsurance Acquisition Expenses	52.7	53.0
Other Underwriting Expense	55.4	56.0
General and administrative expenses	140.5	110.1
Interest expense	5.9	4.3
Total expenses	415.6	391.3
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(0.9)	65.8
Net gain on sale of discontinued operations	0.0	8.0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	0.1	0.0
Comprehensive (loss) income	50.3	25.6
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net change in cash during the period	0.0	7.9
Cash balances at beginning of period	173.5	238.0		$ 238.0
Cash balances at end of period	170.4	180.3			170.4
Net income tax refund from national governments	(13.5)	8.0
Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Total investments	161.8				161.8	686.2
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net realized and unrealized investment gains	4.1	0.3
Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	202.4	205.0
Net investment income	12.2	9.0
Net realized and unrealized investment gains	(8.8)	78.4
Other revenue	(4.1)	(27.4)
Total revenues	201.7	265.0
Loss and loss adjustment expenses	113.7	97.5
Insurance and Reinsurance Acquisition Expenses	48.4	42.6
Other Underwriting Expense	26.7	25.5
General and administrative expenses	8.1	6.3
Interest expense	6.6	6.8
Total expenses	203.5	178.7
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(1.8)	86.3
Discontinued Operation, Tax Effect of Discontinued Operation	0.9	(20.4)
Net gain on sale of discontinued operations	(0.9)	65.9	$ 2.7
Net loss from discontinued operations, net of tax	(0.9)	73.8
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	37.2	(87.6)
Comprehensive (loss) income	36.3	(13.8)
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net cash used for operations - discontinued operations	(50.7)	(14.3)
Net cash (used for) provided from investing activities - discontinued operations	35.0	36.0
Net cash provided from financing activities - discontinued operations	2.4	1.8
Effect of exchange rate changes on cash	4.2	(8.5)
Net change in cash during the period	(9.1)	15.0
Cash balances at beginning of period	143.9	111.5		111.5
Cash balances at end of period	134.8	126.5			134.8
Interest paid	0.0	(12.8)
Net income tax refund from national governments	(36.4)	(7.0)
Discontinued Operations [Member] | Discontinued Operations, Held-for-sale [Member]
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Fixed maturity investments, at fair value	2,758.3				2,758.3	2,374.0
Short-term	534.5				534.5	352.0
Common equities	214.3				214.3	174.4
Other	61.3				61.3	72.2
Total investments	3,568.4				3,568.4	2,972.6
Cash	134.8				134.8	143.9
Reinsurance recoverable on unpaid losses	286.2				286.2	283.1
Reinsurance recoverable on paid losses	11.1				11.1	10.2
Insurance premiums receivable	483.3				483.3	323.6
Deferred acquisition costs	91.6				91.6	74.6
Deferred tax asset	291.2				291.2	303.1
Ceded unearned insurance premiums	134.7				134.7	87.7
Goodwill and intangible assets	10.2				10.2	10.2
Other assets	194.2				194.2	169.0
Assets held for sale	5,215.5				5,215.5	4,407.0
Loss and loss adjustment expense reserves	1,649.5				1,649.5	1,644.4
Unearned insurance premiums	501.3				501.3	342.2
Debt	403.6				403.6	403.5
Deferred tax liability	278.5				278.5	263.6
Accrued incentive compensation	43.1				43.1	63.2
Ceded reinsurance payable	132.4				132.4	67.1
Funds held under insurance contracts	57.0				57.0	52.9
Other liabilities	50.3				50.3	47.1
Total liabilities held for sale	3,121.1				3,121.1	2,884.0
Discontinued Operations [Member] | Sirius Group
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Accounts receivable on unsettled investment sales	9.8				9.8	29.0
Accounts payable on unsettled investment purchases	5.4				5.4	0.0
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Earned insurance premiums	202.4	205.0
Net investment income	12.2	9.0
Net realized and unrealized investment gains	(8.8)	78.4
Other revenue	(4.1)	(27.4)
Total revenues	201.7	265.0
Loss and loss adjustment expenses	113.7	97.5
Insurance and Reinsurance Acquisition Expenses	48.4	42.6
Other Underwriting Expense	26.7	25.5
General and administrative expenses	8.1	6.3
Interest expense	6.6	6.8
Total expenses	203.5	178.7
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(1.8)	86.3
Discontinued Operation, Tax Effect of Discontinued Operation	0.9	(20.4)
Net gain on sale of discontinued operations	(0.9)	65.9
Net loss from discontinued operations, net of tax	(0.9)	65.9
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	37.2	(87.6)
Comprehensive (loss) income	36.3	(21.7)
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Net Assets	2,094.4				2,094.4	$ 1,523.0
Discontinued Operations [Member] | Other Discontinued Operations [Member]
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	0.0	0.0
Total revenues	0.0	0.0
Loss and loss adjustment expenses	0.0	0.0
Insurance and Reinsurance Acquisition Expenses	0.0	0.0
Other Underwriting Expense	0.0	0.0
General and administrative expenses	0.0	0.0
Interest expense	0.0	0.0
Total expenses	0.0	0.0
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	0.0	0.0
Discontinued Operation, Tax Effect of Discontinued Operation	0.0	0.0
Net gain on sale of discontinued operations	0.0	0.0
Net loss from discontinued operations, net of tax	0.0	7.9
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	0.0	0.0
Comprehensive (loss) income	0.0	7.9
Discontinued Operations [Member] | OneBeacon Runoff [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	0.0	(0.1)
Net loss from discontinued operations, net of tax		(0.1)
Discontinued Operations [Member] | Esurance
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net gain on sale of discontinued operations	$ 0.0	$ 8.0		$ 10.3	$ 28.3