Non-controlling Interest (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 460.8	$ 542.7
Total Non-Controlling Excluding Reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 113.3	$ 168.6
OneBeacon
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	24.70%	24.70%
Total non-controlling interests	$ 246.9	$ 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	$ 17.3	$ 17.9
MediaAlpha
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	40.00%	40.00%
Total non-controlling interests	$ 15.1	$ 22.6
Tranzact [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	36.80%	36.80%
Total non-controlling interests	$ 75.6	$ 88.2
WOBI
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	4.70%	36.70%
Total non-controlling interests	$ 1.8	$ 5.4
Prospector Offshore Fund Ltd [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	23.40%
Total non-controlling interests	$ 0.0	$ 31.1
A W G Dewar Inc [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	32.20%	18.00%
Total non-controlling interests	$ 3.5	$ 3.4
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (134.2)	$ (121.9)
Star & Shield Insurance Exchange
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (15.2)	$ (12.4)