Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) (USD $)
In Millions, unless otherwise specified

12 Months Ended

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2010

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

$ (48.4)

$ (41.9)

$ (76.7)

Increase (decrease) in reserves

1,040.5

1,193.9

1,174.3

Accretion of fair value adjustment to net loss and LAE reserves

1.7

10.6

8.3

Un-accreted fair value, adjustment to loss and loss adjustment expense reserves, before tax

4.7

Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

(23.9)

(34.5)

(46.9)

Liability for asbestos and environmental claims, gross

227.8

228.6

207.2

214.3

Liability for asbestos and environmental claims

193.9

189.4

162.7

169.6

Asbestos and Environmental Loss Three Year Survival Ratio Period

8 years

9 years

Liability for asbestos claims, net, period increase (decrease)

11.8

46.4

10.3

Liabilty for environmental claims, net, period increase (decrease)

0.8

(0.5)

2.0

OneBeacon

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

(7.4)

(29.8)

Liability for asbestos and environmental claims, gross

1.0

Chile Earthquake Loss Cover | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Increase (decrease) in reserves

(13.1)

Asbestos | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Liability for asbestos and environmental claims, gross

207.4

208.2

185.1

191.9

Liability for asbestos and environmental claims

178.7

174.2

146.2

151.5

Environmental | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Liability for asbestos and environmental claims, gross

20.4

22.1

22.4

Liability for asbestos and environmental claims

15.2

16.5

18.1

WM Solutions | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Liability for asbestos claims, net, period increase (decrease)

13.0

11.0

Liabilty for environmental claims, net, period increase (decrease)

1.0

0.7

Accident and Health Insurance Product Line [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

(9.2)

Asbestos [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

11.8

46.4

Casualty Runoff [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

(31.5)

Aviation Space [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

(10.1)

(5.3)

Property [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

(16.6)

(28.4)

Property Insurance Product Line [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Increase (decrease) in reserves

(41.2)

asbestos and environmental [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Increase (decrease) in reserves

12.3

Marine Energy [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

(11.6)

Trade Credit [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

(6.8)

Accident and Health | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Loss and LAE incurred relating prior year losses

4.3

Professional Liability | OneBeacon

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Increase (decrease) in reserves

(11.5)

Property and Casualty, Commercial Insurance Product Line [Member] | OneBeacon

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Increase (decrease) in reserves

(14.0)

Other Lines of Business [Member] | OneBeacon

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Increase (decrease) in reserves

(4.3)

Quarterly Monitoring [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Liability for asbestos claims, net, period increase (decrease)

14.0

In-depth Analysis [Member] | Sirius Group

Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity

Liability for asbestos claims, net, period increase (decrease)

$ 33.0