Discontinued Operations (Loss from Discontinued Operations) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended			51 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided from (used for) discontinued operations									$ 14.7	$ 60.0	$ (61.5)
Net cash provided from investing activities — discontinued operations									31.3	35.8	148.7
Revenues
Earned insurance premiums									1,188.2	1,185.0	1,120.9
Net investment income									60.8	59.5	59.8
Other revenue									334.2	131.1	47.0
Net realized and unrealized investment gains									225.4	78.5	133.9
Revenues (including realized gains and losses)	$ 678.5	$ 334.7	$ 403.6	$ 391.8	$ 405.6	$ 323.2	$ 393.5	$ 331.8	1,808.6	1,454.1	1,361.6
Expenses
Loss and loss adjustment expenses									708.9	824.0	622.1
Insurance and reinsurance acquisition expenses									220.1	206.2	210.4
Other underwriting expenses									218.6	179.6	205.2
Interest expense on debt									18.6	15.6	16.2
General and administrative expenses									487.5	257.7	149.1
Total expenses	420.4	429.3	412.7	391.3	471.3	358.7	359.9	293.2	1,653.7	1,483.1	1,203.0
Net income from discontinued operations, net of tax	7.3	$ 14.2	$ 3.5	$ 73.8	82.1	$ 63.9	$ 61.1	$ 51.5	80.6	260.2	99.9
Gain (loss) on sale of discontinued operations, net of tax									18.2	(1.6)	46.6
Change in foreign currency translation, net of tax									(65.8)	(168.5)	2.7
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest									197.2	207.3	247.0
Net cash (used for) provided from financing activities — discontinued operations									(9.1)	(63.2)	(87.5)
Cash and Cash Equivalents, at Carrying Value	173.5				238.0				173.5	238.0	233.5	$ 173.5	$ 118.9
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided from (used for) discontinued operations									14.7	60.0	(61.5)
Net cash provided from investing activities — discontinued operations									31.3	35.8	148.7
Revenues
Earned insurance premiums									847.0	874.0	867.2
Net investment income									40.7	40.9	46.7
Other revenue									(21.3)	(62.4)	27.6
Net realized and unrealized investment gains									15.1	205.4	27.8
Revenues (including realized gains and losses)									881.5	1,057.9	969.3
Expenses
Loss and loss adjustment expenses									422.7	344.6	497.3
Insurance and reinsurance acquisition expenses									189.8	193.7	166.5
Other underwriting expenses									107.9	133.2	125.9
Interest expense on debt									26.6	26.3	26.3
General and administrative expenses									27.0	30.5	32.2
Total expenses									774.0	728.3	848.2
Net income (loss) from discontinued operations									107.5	329.6	121.1
Income tax expense									(26.9)	(69.4)	(21.2)
Net income from discontinued operations, net of tax									80.6	260.2	99.9
Gain (loss) on sale of discontinued operations, net of tax											46.6
Total income from discontinued operations									98.8	258.6	146.5
Change in foreign currency translation, net of tax										(169.5)	6.1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest									33.8	89.1	152.6
Net cash (used for) provided from financing activities — discontinued operations									(9.1)	(63.2)	(87.5)
Effect of Exchange Rate on Cash									(4.5)	(14.3)	(0.2)
Net Cash Provided by (Used in) Discontinued Operations									32.4	18.3	(0.5)
Cash and Cash Equivalents, at Carrying Value	$ 143.9				$ 111.5				143.9	111.5	93.2	143.9	$ 93.7
Sirius Group
Revenues
Net investment income									225.0	4.0	1.0
Net realized and unrealized investment gains									225.0	4.0	1.0
Sirius Group | Discontinued Operations
Revenues
Earned insurance premiums									847.0	873.9	866.4
Net investment income									40.7	40.9	46.7
Other revenue									(20.6)	(62.4)	16.8
Net realized and unrealized investment gains									15.1	205.4	27.8
Revenues (including realized gains and losses)									882.2	1,057.8	957.7
Expenses
Loss and loss adjustment expenses									422.7	345.3	418.4
Insurance and reinsurance acquisition expenses									189.8	193.6	166.5
Other underwriting expenses									107.9	129.7	126.1
Interest expense on debt									26.6	26.3	26.3
General and administrative expenses									27.0	30.5	32.2
Total expenses									774.0	725.4	769.5
Net income (loss) from discontinued operations									108.2	332.4	188.2
Income tax expense									27.1	70.4	46.2
Net income from discontinued operations, net of tax									81.1	262.0	142.0
Gain (loss) on sale of discontinued operations, net of tax											0.0
Total income from discontinued operations									81.1	262.0	142.0
Change in foreign currency translation, net of tax									(65.0)	(169.5)	6.1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest									16.1	92.5	148.1
Other Operations | Discontinued Operations
Revenues
Earned insurance premiums									0.0	0.1	0.8
Net investment income									0.0	0.0	0.0
Other revenue									(0.7)	0.0	10.8
Net realized and unrealized investment gains									0.0	0.0	0.0
Revenues (including realized gains and losses)									(0.7)	0.1	11.6
Expenses
Loss and loss adjustment expenses									0.0	(0.7)	78.9
Insurance and reinsurance acquisition expenses									0.0	0.1	0.0
Other underwriting expenses									0.0	3.5	(0.2)
Interest expense on debt									0.0	0.0	0.0
General and administrative expenses									0.0	0.0	0.0
Total expenses									0.0	2.9	78.7
Net income (loss) from discontinued operations									(0.7)	(2.8)	(67.1)
Income tax expense									(0.2)	(1.0)	(25.0)
Net income from discontinued operations, net of tax									(0.5)	(1.8)	(42.1)
Total income from discontinued operations									17.7	(3.4)	4.5
Change in foreign currency translation, net of tax									(65.0)	0.0	0.0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest									17.7	(3.4)	4.5
Fireman's Fund Insurance Company [Member] | Discontinued Operations
Expenses
Gain (loss) on sale of discontinued operations, net of tax										14.0
OneBeacon Runoff | Discontinued Operations
Expenses
Gain (loss) on sale of discontinued operations, net of tax									0.3	(18.8)	46.6
Esurance [Member] | Discontinued Operations
Expenses
Net income from discontinued operations, net of tax											4.5
Gain (loss) on sale of discontinued operations, net of tax									17.9	3.2		$ 28.3
Other Segments
Revenues
Earned insurance premiums									8.7	6.1	0.0
Net investment income									8.8	9.0	11.1
Other revenue									334.1	124.7	15.4
Net realized and unrealized investment gains									259.9	29.8	95.8
Revenues (including realized gains and losses)									611.5	169.6	122.3
Expenses
Loss and loss adjustment expenses									8.2	8.9	0.0
Insurance and reinsurance acquisition expenses									3.4	0.8	0.0
Other underwriting expenses									0.0	0.0	0.0
Interest expense on debt									5.6	2.6	3.2
Total expenses									452.5	218.7	$ 106.4
Symetra | Sirius Group
Revenues
Net realized and unrealized investment gains									218.5
Symetra | Other Segments
Revenues
Net realized and unrealized investment gains									258.8
Tranzact [Member] | Other Segments
Revenues
Other revenue									186.9	43.2
Expenses
General and administrative expenses									167.3	37.4
MediaAlpha [Member] | Other Segments
Revenues
Other revenue									105.5	65.3
Expenses
General and administrative expenses									$ 99.0	$ 60.6