Non-controlling Interest (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 451.4	$ 454.8
Total Non-Controlling Excluding Reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 114.8	$ 115.2
OneBeacon [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	23.90%	24.50%
Total non-controlling interests	$ 243.4	$ 245.6
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	$ 17.2	$ 17.1
MediaAlpha [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	40.00%	40.00%
Total non-controlling interests	$ 14.2	$ 14.4
Tranzact [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	36.80%	36.80%
Total non-controlling interests	$ 79.7	$ 79.4
WOBI
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.90%	3.90%
Total non-controlling interests	$ 0.5	$ 0.6
AWG Dewar
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	30.10%	19.00%
Total non-controlling interests	$ 3.2	$ 3.7
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (140.7)	$ (140.0)
Star & Shield Insurance Exchange
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (16.1)	$ (16.0)