United Airlines handed travelers an unexpected bonus Wednesday, announcing that reward miles will no longer expire.

Under the airline's MileagePlus program, miles had previously expired after 18 months with no activity. As of Wednesday, the carrier joined Delta and JetBlue, which also offer miles with no expiration date.

"We want to demonstrate to our members that we are committing to them for the long haul, and giving customers a lifetime to use miles is an exceptionally meaningful benefit," Luc Bondar, United's vice president of loyalty and president of MileagePlus, said in a statement.

While United's announcement said the new policy was meant to give members "a lifetime to use miles," spokeswoman Maddie King said those miles could even outlive some people, since there are no hard rules dictating what happens to miles after someone dies. "We work with family members who have lost loved ones to figure out the best way to assist them with MileagePlus accounts and miles," she said in an email.

The change leaves American Airlines and Southwest as the largest U.S. carriers with miles or points that expire if travelers don't fly or take other action that would keep their accounts active. Miles on American expire after 18 months of inactivity; at Southwest, points expire in 24 months. Alaska Airlines, a smaller carrier, also warns that inactive accounts expire after two years and miles are deleted.

Industry watchers said they expect to see other airlines follow United's new move.

James Larounis, a travel industry analyst who writes about points and miles for UpgradedPoints.com, cautioned that although many customers hold onto their miles to save for a big trip, that's not always the best strategy, whether points expire or not.

"It's never a good idea to continue to save miles for a goal months or years down the road, as at any point in between, the airline could devalue that currency, Larounis said. "By extending the life of these miles, it's a huge marketing ploy."

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