Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of property and casualty insurer White Mountains Insurance (NYSE: WTM) soared as much as 16% in intraday trading on heavier-than-average volume.

So what: Insurance giant Allstate (NYSE: ALL) agreed to buy Esurance and Answer Financial from White Mountains in a transaction that is expected to end up right around $1 billion. For Allstate, the purchase gives the company a bit more traction in its quest to catch up with the online presence of competitors such as Berkshire Hathaway's (NYSE: BRK-A) (NYSE: BRK-B) Geico and Progressive (NYSE: PGR). Though Esurance is a relatively small player, it will give Allstate's efforts a definite jump-start.

Now what: From White Mountains' perspective, the transaction may be pretty smart. While Esurance is a valuable asset, White Mountains investors may be getting a more fully realized price for it through the sale. In its press release, the company said it expects its adjusted per-share book value will increase by $80 thanks to the deal. In addition, the vanilla-policy, consumer-facing style of Esurance and Answer Financial are fairly different from White Mountains' primary insurance businesses, so the company may see this as a way to focus on its core competencies.

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