Non-controlling Interest (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 528.5	$ 542.7
Total Non-Controlling Excluding Reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	160.2	168.6
OneBeacon Ltd.
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	24.70%	24.70%
Total non-controlling interests	258.8
OneBeacon
Noncontrolling Interest [Line Items]
Total non-controlling interests	258.8	258.4
SIG Preference Shares
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	250.0	250.0
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	17.7	17.9
QuoteLab
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	40.00%	40.00%
Total non-controlling interests	22.0	22.6
Tranzact [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	36.80%	36.80%
Total non-controlling interests	81.8	88.2
WOBI
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	36.70%	36.70%
Total non-controlling interests	4.7	5.4
Prospector Offshore Fund Ltd [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	32.40%	23.40%
Total non-controlling interests	31.0	31.1
A W G Dewar Inc [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	20.30%	18.00%
Total non-controlling interests	3.0	3.4
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	(127.0)	(121.9)
Star & Shield Insurance Exchange
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (13.5)	$ (12.4)