One thing to be aware of, if you're a non-U.S. person, is that there is a U.S. estate tax that applies to all U.S. broker held cash, in any/all currencies. There is a single US$60,000 estate tax exemption that applies to all your U.S. estate taxable assets.
So, here's how this works. Let's suppose you were to die on June 29, 2018. (Hopefully not! It's just an example.) Let's also suppose you own 100 shares of General Electric (a U.S. listed/traded stock) held in a custodial account that your employer (GE) set up for you, and you have US$50,000 and S$35,000 of cash held at Interactive Brokers. You also have US$40,000 worth of U.S. Treasury Bills, purchased through Interactive Brokers. And you have US$20,000 worth of IWDA, again at IB. Plus some assets in Singapore, such as some equity in a HDB unit and CPF savings.
OK, got all that? So (oversimplifying only slightly), the way the U.S. estate tax works for non-U.S. persons is that all your assets are tallied up, at fair market value, on the date of your death. Some assets are U.S. estate taxable, and some are not. In this particular list, the following assets are U.S. estate taxable:
* 100 shares of GE (worth about US$1,320 right now)
* US$50,000 cash at IB
* S$35,000 cash at IB (worth about US$29,915 right now)
Total = US$81,235
and that's it. To this list of U.S. estate taxable assets a single US$60,000 exemption is applied. The value of your U.S. estate taxable assets above the single exemption (about US$21,235) are then assessed a 40% estate tax, and the executor of your estate (the person responsible for settling your financial affairs after you die, whoever that is) then is required to pay, under U.S. law, that estate tax (with accompanying estate tax form) within 9 months of your death. U.S. estate tax owed in this example would be about US$8,494.
Provided you keep your U.S. estate taxable assets at or under the US$60,000 single exemption, no U.S. estate tax will be owed in the event of your demise.