Investment Securities (Change in net unrealized gains or losses for Level 3 assets) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	$ 11.6	[1]	$ 4.5
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	4.7		(4.4)
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	0.7		(0.9)
Other investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	3.1		1.9
Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	3.4		2.0
Level 3 | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	0.3		0.1
Level 3 | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	0		0
Level 3 | Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	0		0
Level 3 | Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	0		0
Level 3 | Other investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	$ 3.1		$ 1.9

[1] Includes pre-tax changes in net deferred gains and losses on sales of investments between OneBeacon and entities under White Mountains' common control of $(0.1) million for the three months ended March 31, 2013, with none recorded in the 2014 period.