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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Fool contributor Matt Koppenheffer owns shares of Berkshire Hathaway, but does not have a financial interest in any of the other companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.