Held for Sale and Discontinued Operations (Loss from Discontinued Operations) (Details) - USD ($) $ in Millions		3 Months Ended									9 Months Ended	12 Months Ended
	Sep. 28, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Earned insurance premiums												$ 13.9	$ 10.4	$ 13.4
Net investment income												59.0	56.0	32.1
Other revenue												10.2	7.1	21.3
Net realized and unrealized investment gains												(108.3)	133.3	(27.4)
Revenues (loss) (including realized and unrealized gains and losses)		$ 6.0	$ 198.7	$ 122.3	$ 42.1	$ 114.0	$ 87.5	$ 83.5	$ 88.8			369.1	373.8	157.7
Expenses
Loss and loss adjustment expenses													1.1	8.0
Other underwriting expenses												0.4	0.4	0.4
General and administrative expenses												235.7	208.0	175.5
Interest expense on debt												9.5	2.3	3.0
Total expenses		150.1	154.4	134.7	108.1	108.8	79.1	85.7	92.4			547.3	366.0	305.0
Net income from discontinued operations, net of tax		$ 0.0	$ (17.3)	$ 0.0	$ 0.1	$ 4.3	$ 539.1	$ 2.8	$ 31.3			0.0	20.5	108.3
Gain (loss) on sale of other discontinued operations, net of tax												(17.2)	557.0	415.1
Total income (loss) from discontinued operations												(17.2)	577.5	523.4
Comprehensive income from discontinued operations												(146.0)	630.7	547.4
Discontinued Operations
Expenses
Total income (loss) from discontinued operations												(17.2)	577.5	523.4
OneBeacon
Expenses
Net income from discontinued operations, net of tax											$ 20.5			108.6
Gain (loss) on sale of other discontinued operations, net of tax	$ 554.6
OneBeacon | Discontinued Operations
Revenues
Earned insurance premiums													807.6	1,100.6
Net investment income													39.7	50.6
Other revenue													7.7	5.5
Net realized and unrealized investment gains													38.8	37.7
Revenues (loss) (including realized and unrealized gains and losses)													893.8	1,194.4
Expenses
Loss and loss adjustment expenses													546.0	656.0
Insurance and reinsurance acquisition expenses													145.6	206.0
Other underwriting expenses													156.2	209.0
General and administrative expenses													21.2	14.2
Interest expense on debt													10.0	13.1
Total expenses													879.0	1,098.3
Net income from discontinued operations													14.8	96.1
Income tax benefit													5.7	12.5
Net income from discontinued operations, net of tax													20.5	108.6
Gain (loss) on sale of other discontinued operations, net of tax												0.0	554.5	0.0
Total income (loss) from discontinued operations													575.0	108.6
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax													0.3	1.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax													2.9	0.0
Comprehensive income from discontinued operations													578.2	109.6
Sale of Sirius Group
Expenses
Recognition of foreign currency translation from sale of Sirius Group, net of tax														113.3
Sale of Sirius Group | Discontinued Operations
Expenses
Net income from discontinued operations, net of tax														(4.3)
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax														32.0
Sirius Group
Revenues
Net realized and unrealized investment gains														3.7
Expenses
Gain (loss) on sale of other discontinued operations, net of tax										$ (4.0)				363.2
Recognition of foreign currency translation from sale of Sirius Group, net of tax														113.3
Sirius Group | Discontinued Operations
Revenues
Earned insurance premiums													0.0	240.1
Net investment income													0.0	14.4
Other revenue													0.0	0.6
Net realized and unrealized investment gains													0.0	(1.5)
Revenues (loss) (including realized and unrealized gains and losses)													0.0	253.6
Expenses
Loss and loss adjustment expenses													0.0	154.9
Insurance and reinsurance acquisition expenses													0.0	59.0
Other underwriting expenses													0.0	30.9
General and administrative expenses													0.0	10.4
Interest expense on debt														7.9
Total expenses													0.0	263.1
Net income from discontinued operations													0.0	(9.5)
Income tax benefit													0.0	3.1
Net income from discontinued operations, net of tax													0.0	(6.4)
Gain (loss) on sale of other discontinued operations, net of tax												(17.2)	(0.7)	363.2
Total income (loss) from discontinued operations													(0.7)	356.8
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax													0.0	32.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax													0.0	113.3
Comprehensive income from discontinued operations													(0.7)	502.1
Tranzact
Expenses
Income tax benefit														(30.2)
Gain (loss) on sale of other discontinued operations, net of tax													3.2	51.9
Tranzact | Discontinued Operations
Revenues
Earned insurance premiums													0.0	0.0
Net investment income													0.0	0.0
Other revenue													0.0	119.6
Net realized and unrealized investment gains													0.0	0.0
Revenues (loss) (including realized and unrealized gains and losses)													0.0	119.6
Expenses
Loss and loss adjustment expenses													0.0	0.0
Insurance and reinsurance acquisition expenses													0.0	0.0
Other underwriting expenses													0.0	0.0
General and administrative expenses													0.0	116.7
Interest expense on debt														3.2
Total expenses													0.0	119.9
Net income from discontinued operations													0.0	(0.3)
Income tax benefit													0.0	6.4
Net income from discontinued operations, net of tax													0.0	6.1
Gain (loss) on sale of other discontinued operations, net of tax												$ 0.0	3.2	51.9
Total income (loss) from discontinued operations													3.2	58.0
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax													0.0	0.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax													0.0	0.0
Comprehensive income from discontinued operations													3.2	58.0
Total | Discontinued Operations
Revenues
Earned insurance premiums													807.6	1,340.7
Net investment income													39.7	65.0
Other revenue													7.7	125.7
Net realized and unrealized investment gains													38.8	36.2
Revenues (loss) (including realized and unrealized gains and losses)													893.8	1,567.6
Expenses
Loss and loss adjustment expenses													546.0	810.9
Insurance and reinsurance acquisition expenses													145.6	265.0
Other underwriting expenses													156.2	239.9
General and administrative expenses													21.2	141.3
Interest expense on debt													10.0	24.2
Total expenses													879.0	1,481.3
Net income from discontinued operations													14.8	86.3
Income tax benefit													5.7	22.0
Net income from discontinued operations, net of tax													20.5	108.3
Gain (loss) on sale of other discontinued operations, net of tax													557.0	415.1
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax													0.3	33.0
Recognition of foreign currency translation from sale of Sirius Group, net of tax													2.9	113.3
Comprehensive income from discontinued operations													$ 580.7	$ 669.7