Derivatives (Details) ¥ in Billions, $ in Billions	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	$ 0.4	¥ 50.7
Collective account values of underlying variable annuities as a percentage of the guarantee value	109.00%	109.00%

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant