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, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				$ (4.0)	$ 92.0	$ 0.0
Increase (decrease) in reserves				708.9	824.0	622.1
Current Accident Year				712.9	732.0	$ 622.1
OneBeacon
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year	$ 75.5	$ 75.5	$ 14.3		75.5
Increase (decrease) in reserves		109.2	109.2	1.8	89.8
Current Accident Year		33.7	$ 33.7
Star & Shield Insurance Exchange [Member]
Loss and Loss Adjustment Expense Reserve Activity
Prior Accident Year				$ 2.2	2.2
Professional Insurance | 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 | 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