Derivatives (Details) ¥ in Billions, $ in Billions	Jun. 30, 2015 USD ($)	Jun. 30, 2015 JPY (¥)	Dec. 31, 2014 USD ($)	Dec. 31, 2014 JPY (¥)
Collective account values of underlying variable annuities as a percentage of the guarantee value			113.00%	113.00%
White Mountains Life Re
Notional amount	$ 0.8	¥ 100.7	$ 1.1	¥ 134.2
Collective account values of underlying variable annuities as a percentage of the guarantee value	113.00%	113.00%