• At least 2 million new customers visited Kohl's stores thanks to an Amazon returns service that it offers nationwide.
• The company said about a third of those customers are millennials.
• "While details of the partnership are confidential, we will continue to see that this is accretive to both sales and profit," Chief Executive Officer Michelle Gass said.

Kohl's said it added at least 2 million new customers last year thanks to an unlikely friend that it has found in Amazon.

"While details of the partnership are confidential, we will continue to see that this is accretive to both sales and profit," Chief Executive Officer Michelle Gass said during a conference call Tuesday morning.

Kohl's began accepting Amazon returns at 10 of its stores in the spring of 2017. By the summer of 2019, the Amazon returns kiosks were rolled out nationwide. The hope has been that the service would drive foot traffic and boost sales.

According to Kohl's, a third of these new visitors were millennials. The company likely hopes it can make these new shoppers repeat visitors.

Conversion rates are trending upward for the people visiting Kohl's stores to return a package to Amazon, Gass said, which means visitors are increasingly ringing something up at the register before they go.

Gass has been looking to collaborations with other big-name brands to broaden Kohl's customer base, including an upcoming partnership with the beauty business Sephora.

Earlier Tuesday, Kohl's reported fiscal fourth-quarter earnings and sales that topped analysts' estimates. The company predicts stronger growth ahead. It also reinstated its dividend and said it plans to buy back shares.

Meanwhile, Kohl's is also facing pressure from a group of activist investors who have attempted to seize control of the company's board. Among other things, the group has said it's skeptical that Kohl's tie-up with Amazon is adding to sales. In a letter published in late February, the activists called out Kohl's "reluctance to share detailed information about the program with investors."

Kohl's shares recently were up more than 2%. Its stock has gained 45% over the past 12 months, as of Monday's market close, bringing its market value to $8.99 billion. That's bigger than rival Nordstrom's and Macy's.