Investment Securities (Changes in fair value) (Details) (USD $)	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	$ 11,600,000	[1]	$ 4,500,000
Changes in net foreign currency translation gains (losses)	0	[1]	0	[1]
Total net changes in fair value reflected in revenues	11,600,000	[1]	4,500,000	[1]
Change in Deferred Gains (Losses)	0		(100,000)
Fixed maturity investments
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	4,700,000		(4,400,000)
Changes in net foreign currency translation gains (losses)	0		0	[1]
Total net changes in fair value reflected in revenues	4,700,000	[1]	(4,400,000)	[1]
Common equity securities
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	3,100,000		7,900,000
Changes in net foreign currency translation gains (losses)	0		0	[1]
Total net changes in fair value reflected in revenues	3,100,000	[1]	7,900,000	[1]
Convertible fixed maturity investments
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	700,000		(900,000)
Changes in net foreign currency translation gains (losses)	0		0	[1]
Total net changes in fair value reflected in revenues	700,000	[1]	(900,000)	[1]
Other investments
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	3,100,000		1,900,000
Changes in net foreign currency translation gains (losses)	0
Total net changes in fair value reflected in revenues	$ 3,100,000	[1]	$ 1,900,000	[1]

[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.