It was not a scenario the Allstate actuaries envisioned, in pricing policies that would generate more than $300 million in premiums from Connecticut drivers over a 12-month period.

But with many vehicles languishing in garages as a result of “stay-at-home” orders, the insurance giant pledged Monday to do right for its policyholders nationally by refunding $600 million in premiums that were earmarked to cover auto accidents this spring that will never come to be.

American Family Insurance is following suit, pledging on Monday a $200 million refund, with the Wisconsin-based underwriter having a comparatively small Connecticut customer base at under $8 million in premiums collected in 2017, the most recent year for which the Connecticut Insurance Department has posted data.

As a group, auto insurers collected nearly $3.5 billion in premiums from Connecticut households and businesses in 2017, working out to an average monthly tally of $288 million.

The Geico subsidiary of Berkshire Hathaway is the largest auto insurance underwriter in Connecticut, with a $580 million book of business in 2017. Progressive and Liberty Mutual joined Allstate in logging more than $300 million in Connecticut premiums that year, with Travelers and State Farm topping the $200 million threshold.

Nearly two weeks ago, Geico provided a needed relief valve for many customers, stating it would not cancel coverage through April 30 whether due to non-payment or the expiration of policies. Travelers, which is a major Hartford-area employer, set a May 15 date through which it will not cancel policies, while taking separate action on behalf of the independent brokers that steer it business in accelerating the payment of $100 million in commissions.

Customers of Allstate and subsidiaries Esurance and Encompass will receive a 15 percent refund of their monthly premiums for both April and May, as credits to their bank accounts, credit cards or Allstate accounts.

Alex.Soule@scni.com; 203-842-2545; @casoulman