Derivatives (Details) ¥ in Billions, $ in Billions	Mar. 31, 2016 USD ($)	Mar. 31, 2016 JPY (¥)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 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	$ 0.2	¥ 21.2	$ 0.4	¥ 50.7
Collective account values of underlying variable annuities as a percentage of the guarantee value	107.00%	107.00%	109.00%	109.00%