Non-controlling Interest (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (131.6)	$ 133.3
Total Non-Controlling Excluding Reciprocals
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 28.6	$ 35.2
OneBeacon
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	23.90%
Total non-controlling interests	$ 0.0	$ 244.6
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	$ 16.1	$ 16.6
MediaAlpha
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	40.00%	40.00%
Total non-controlling interests	$ 9.5	$ 11.7
Buzzmove
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	22.90%	29.10%
Total non-controlling interests	$ 3.0	$ 2.9
WOBI
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	5.00%	5.00%
Total non-controlling interests	$ 0.0	$ 0.1
AWG Dewar
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	18.80%
Total non-controlling interests	$ 0.0	$ 3.9
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (160.2)	$ (150.9)
Star & Shield Insurance Exchange
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	100.00%
Total non-controlling interests	$ 0.0	$ 4.4
Star & Shield Insurance Exchange | Total mutual and reciprocals
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (160.2)	$ (146.5)