Any person who engages in a continuing criminal enterprise shall be sentenced to a term of imprisonment which may not be less than 20 years and which may be up to life imprisonment, to a fine not to exceed the greater of that authorized in accordance with the provisions of title 18 or $2,000,000 if the defendant is an individual or $5,000,000 if the defendant is other than an individual, and to the forfeiture prescribed in section 853 of this title; except that if any person engages in such activity after one or more prior convictions of him under this section have become final, he shall be sentenced to a term of imprisonment which may not be less than 30 years and which may be up to life imprisonment, to a fine not to exceed the greater of twice the amount authorized in accordance with the provisions of title 18 or $4,000,000 if the defendant is an individual or $10,000,000 if the defendant is other than an individual, and to the forfeiture prescribed in section 853 of this title.