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SCHEDULE IV REINSURANCE
 12 Months Ended
Dec. 31, 2012
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]  
SCHEDULE IV REINSURANCE
WHITE MOUNTAINS INSURANCE GROUP, LTD.
REINSURANCE
(Millions)

Column A
 
Column B
 
Column C
 
Column D
 
Column E
 
Column F
Premiums earned
 
Gross amount
 
Ceded to other
companies
 
Assumed from
other companies
 
Net amount
 
Percentage of
amount assumed
to net
Years ended:
 
 

 
 

 
 

 
 

 
 

December 31, 2012
 
 

 
 

 
 

 
 

 
 

OneBeacon
 
$
1,158.3

 
$
(79.1
)
 
$
52.8

 
$
1,132.0

 
4.7
%
Sirius Group
 
169.9

 
(226.6
)
 
988.3

 
931.6

 
106.1
%
December 31, 2011
 
 

 
 

 
 

 
 

 
 

OneBeacon
 
$
1,035.9

 
$
(66.0
)
 
$
42.3

 
$
1,012.2

 
4.2
%
Sirius Group
 
128.5

 
(206.0
)
 
989.8

 
912.3

 
108.5
%
December 31, 2010
 
 

 
 

 
 

 
 

 
 

OneBeacon
 
$
1,242.5

 
$
(122.1
)
 
$
60.7

 
$
1,181.1

 
5.1
%
Sirius Group
 
117.9

 
(199.7
)
 
929.7

 
847.9

 
109.6
%