Ameren (AEE) said late Monday that it started a $520 million underwritten offering of shares.

All shares are expected to be borrowed by forward counterparties from third parties and sold to its underwriters and offered in connection with the forward sale agreements, it said.

Ameren said it expects to enter into forward sale agreements with some underwriters, each in its capacity as forward counterparty.

Underwriters in the offering will have a 30-day option to buy up to an additional $78 million shares, according to Ameren.