Discontinued Operations (Loss from Discontinued Operations) (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues
Earned insurance premiums										$ 1,114.0	$ 1,188.2	$ 1,185.0
Net investment income										86.8	60.8	59.5
Net realized and unrealized investment gains										10.3	225.4	78.5
Other revenue										149.6	147.3	88.1
(Loss) revenues (including realized gains and losses)	$ 268.2	$ 365.0	$ 358.2	$ 369.3	$ 602.6	$ 296.1	$ 368.7	$ 354.3		1,360.7	1,621.7	1,411.1
Expenses
Loss and loss adjustment expenses										664.0	708.9	824.0
Insurance acquisition expenses										211.6	220.1	206.2
Other underwriting expenses										209.5	218.6	179.6
Interest expense on debt										16.1	14.6	14.2
General and administrative expenses										299.9	302.2	216.5
Total expenses	335.0	345.0	360.3	360.8	356.3	383.0	373.8	351.3		1,401.1	1,464.4	1,440.5
Net (loss) income from discontinued operations, net of tax	$ (6.8)	61.9	358.6	$ 1.1	$ 19.6	$ 6.5	$ (0.5)	$ 71.3		(0.3)	78.7	260.6
Gain (loss) from sale of other discontinued operations, net of tax										415.1	18.2	(1.6)
OCI, Change in foreign currency transaction and translation adjustment, net of tax										0.3	(0.5)	(10.7)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax											0.0	0.0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest										558.1	197.2	207.3
Discontinued Operations
Revenues
Earned insurance premiums										240.1	847.0	874.0
Net investment income										14.4	40.7	40.9
Net realized and unrealized investment gains										(1.5)	15.1	205.4
Other revenue										120.2	165.6	(19.4)
(Loss) revenues (including realized gains and losses)										373.2	1,068.4	1,100.9
Expenses
Loss and loss adjustment expenses										154.9	422.7	344.6
Insurance acquisition expenses										59.0	189.8	193.7
Other underwriting expenses										30.9	107.9	133.2
Interest expense on debt										11.1	30.6	27.7
General and administrative expenses										127.1	212.3	71.7
Total expenses										383.0	963.3	770.9
Net (loss) income from discontinued operations										(9.8)	105.1	330.0
Income tax benefit										9.5	(26.4)	(69.4)
Net (loss) income from discontinued operations, net of tax										(0.3)	78.7	260.6
Total income from discontinued operations										414.8	96.9	259.0
OCI, Change in foreign currency transaction and translation adjustment, net of tax											(65.0)	(169.5)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest										560.1	31.9	89.5
Sirius Group
Revenues
Net realized and unrealized investment gains										3.7	205.0	2.7
Expenses
Gain (loss) from sale of other discontinued operations, net of tax			$ 363.2							363.2
Sirius Group | Discontinued Operations
Revenues
Earned insurance premiums										240.1	847.0	873.9
Net investment income										14.4	40.7	40.9
Net realized and unrealized investment gains										(1.5)	15.1	205.4
Other revenue										0.6	(20.6)	(62.4)
(Loss) revenues (including realized gains and losses)										253.6	882.2	1,057.8
Expenses
Loss and loss adjustment expenses										154.9	422.7	345.3
Insurance acquisition expenses										59.0	189.8	193.6
Other underwriting expenses										30.9	107.9	129.7
Interest expense on debt										7.9	26.6	26.3
General and administrative expenses										8.3	27.0	30.5
Total expenses										261.0	774.0	725.4
Net (loss) income from discontinued operations										(7.4)	108.2	332.4
Income tax benefit										3.1	(27.1)	(70.4)
Net (loss) income from discontinued operations, net of tax										(4.3)	81.1	262.0
Gain (loss) from sale of other discontinued operations, net of tax										363.2		0.0
Total income from discontinued operations										358.9	81.1	262.0
OCI, Change in foreign currency transaction and translation adjustment, net of tax										32.0	(65.0)	(169.5)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax										113.3
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest										504.2	16.1	92.5
Tranzact
Expenses
Income tax benefit		(30.2)
Gain (loss) from sale of other discontinued operations, net of tax		$ 51.9								51.9
Tranzact | Discontinued Operations
Expenses
Income tax benefit										(8.8)
Net (loss) income from discontinued operations, net of tax										6.1	(1.9)	0.4
Gain (loss) from sale of other discontinued operations, net of tax										51.9
Other Operations | Discontinued Operations
Revenues
Earned insurance premiums										0.0	0.0	0.1
Net investment income										0.0	0.0	0.0
Net realized and unrealized investment gains										0.0	0.0	0.0
Other revenue										119.6	186.2	43.0
(Loss) revenues (including realized gains and losses)										119.6	186.2	43.1
Expenses
Loss and loss adjustment expenses										0.0	0.0	(0.7)
Insurance acquisition expenses										0.0	0.0	0.1
Other underwriting expenses										0.0	0.0	3.5
Interest expense on debt										3.2	4.0	1.4
General and administrative expenses										118.8	185.3	41.2
Total expenses										122.0	189.3	45.5
Net (loss) income from discontinued operations										(2.4)	(3.1)	(2.4)
Income tax benefit										6.4	0.7	1.0
Net (loss) income from discontinued operations, net of tax										4.0	(2.4)	(1.4)
Total income from discontinued operations										55.9	15.8	(3.0)
OCI, Change in foreign currency transaction and translation adjustment, net of tax											0.0	0.0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest										55.9	15.8	(3.0)
Fireman's Fund Insurance Company [Member] | Discontinued Operations
Expenses
Gain (loss) from sale of other discontinued operations, net of tax												14.0
OneBeacon Runoff
Expenses
Gain (loss) from sale of other discontinued operations, net of tax									$ 36.1
OneBeacon Runoff | Discontinued Operations
Expenses
Gain (loss) from sale of other discontinued operations, net of tax											0.3	(18.8)
Total income from discontinued operations											(0.5)
Esurance [Member]
Expenses
Gain (loss) from sale of other discontinued operations, net of tax										0.0	17.9	3.2
Esurance [Member] | Discontinued Operations
Expenses
Gain (loss) from sale of other discontinued operations, net of tax										28.3	17.9	3.2
Other Segments
Revenues
Earned insurance premiums										7.5	8.7	6.1
Net investment income										27.2	8.8	9.0
Net realized and unrealized investment gains										(28.1)	259.9	29.8
Other revenue										143.0	147.2	81.7
(Loss) revenues (including realized gains and losses)										149.6	424.6	126.6
Expenses
Loss and loss adjustment expenses										8.0	8.2	8.9
Insurance acquisition expenses										2.2	3.4	0.8
Other underwriting expenses										0.1	0.0	0.0
Interest expense on debt										3.0	1.6	1.2
Total expenses										259.4	263.2	176.1
Symetra | Other Segments
Revenues
Net realized and unrealized investment gains											258.8
MediaAlpha [Member] | Other Segments
Revenues
Other revenue										116.5	105.5	65.3
Expenses
General and administrative expenses										$ 109.6	$ 99.0	$ 60.6