Derivatives	3 Months Ended
Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	Derivatives White Mountains’s investment portfolio includes investments denominated in Japanese Yen, Euros, GBP and other foreign currencies. White Mountains previously entered into foreign currency forward contracts to manage its foreign currency exposure related to these investments. In the first quarter of 2018, in conjunction with the liquidation of the GBP investment grade corporate bond mandate, White Mountains closed the associated foreign currency forward contract. White Mountains no longer has any open foreign currency forward contracts. As of December 31, 2017, White Mountains held $206.3 million total gross notional value of a foreign currency forward contract with a carrying value of $(3.7) million. The derivative (losses) gains recognized in net realized and unrealized investment gains (losses) for the three months ended March 31, 2018 and 2017 were $(3.5) million and $3.0 million.

Derivatives

3 Months Ended

Mar. 31, 2018

Derivative Instruments and Hedging Activities Disclosure [Abstract]

White Mountains’s investment portfolio includes investments denominated in Japanese Yen, Euros, GBP and other foreign currencies. White Mountains previously entered into foreign currency forward contracts to manage its foreign currency exposure related to these investments. In the first quarter of 2018, in conjunction with the liquidation of the GBP investment grade corporate bond mandate, White Mountains closed the associated foreign currency forward contract. White Mountains no longer has any open foreign currency forward contracts. As of December 31, 2017, White Mountains held $206.3 million total gross notional value of a foreign currency forward contract with a carrying value of $(3.7) million.

The derivative (losses) gains recognized in net realized and unrealized investment gains (losses) for the three months ended March 31, 2018 and 2017 were $(3.5) million and $3.0 million.