Derivatives (Details) ¥ in Billions, $ in Billions	Sep. 30, 2015 USD ($)	Sep. 30, 2015 JPY (¥)	Dec. 31, 2014 USD ($)	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	$ 0.6	¥ 76.3	$ 1.1	¥ 134.2
Collective account values of underlying variable annuities as a percentage of the guarantee value	107.00%	107.00%