Conn. Gen. Stat. § 12-204f

(b) If any domestic insurance company has filed its tax return under this chapter for the calendar year on or before the due date of such return or, if an extension of time to file has been requested and granted, on or before the extended due date of such return, any overpayment reported on such return, if the company has elected to credit such overpayment against the company's estimated tax for the succeeding calendar year, shall be treated as if paid on the due date of the first required installment of estimated tax for such succeeding calendar year. Such overpayment shall be credited against the otherwise unpaid required installments in the order in which such installments are required to be paid under section 12-204c .