Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Loss and Loss Adjustment Expense Reserve Activity
Workers compensation discount percent				2.50%	2.50%
Prior Accident Year				$ 14.8	$ (4.0)	$ 92.0
Increase (decrease) in reserves				664.0	708.9	824.0
Current Accident Year				649.2	712.9	732.0
Workers compensation discount amount				1.6	1.1
OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	$ 75.5	$ 75.5	$ 14.3	15.4	(1.8)	89.8
Increase (decrease) in reserves		109.2	109.2			89.8
Current Accident Year		33.7	$ 33.7
Star & Shield Insurance Exchange [Member]
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				(0.6)	$ (2.2)	2.2
OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Workers compensation discount amount				$ 1.6
Professional Insurance (1) | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year		46.4
Increase (decrease) in reserves		69.3				59.1
Current Accident Year		22.9
Crop Business | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year		0.0
Increase (decrease) in reserves		3.8				0.0
Current Accident Year		3.8
Specialty Products | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year		51.7
Increase (decrease) in reserves		80.1				$ 61.8
Current Accident Year		28.4
Management's estimate [Member] | Specialty Products | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Current Accident Year		$ 29.9
Minimum | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Years that Claims Close				5 years
Maximum | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Years that Claims Close				7 years
Healthcare Sector [Member] | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				$ 40.7
Technology Sector [Member] | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				9.3
Programs [Member] | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				13.3
Accident | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				16.1
Entertainment | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				9.4
Financial Services Sector | OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				$ 8.2