TD waterhouse merged with Ameritrade (previously Datek) a few years ago. so it's the same thing, now they officially use TD Ameritrade. check https://www.tdameritrade.com/welcome1.html for detail.
be aware to pay attention to the fee they charge, they usually charge more service fee (transaction fee) than in Taiwan. good thing is if you are not American citizen, you don't pay capital gain tax.