Quote from raleigh1208:
It is my understanding from talking with IB representatives that if you have an individual account and a joint account with your spouse, those are considered two accounts, and hence SIPC protection of $500,000 for each account. A separate IRA account should be a separate account for SIPC and insurance purposes. I would assume an individual cash account and an individual margin account for the same individual would be combined, so only one SIPC protection of $500,000.
IB's insurance policy with Lloyd's of London is for an additional $29.5 million, and I assume accounts would be determined the same way as accounts are determined for the SIPC protection. The Lloyd's policy has an aggregate limit of $150 million. So while each customer account would be insured up to the $29.5 million under the Lloyd's policy, there's an overall limit of $150 million.
So if in the unlikely event that IB would go belly up, then customers could claim the $500,000 SIPC coverage, as long as SIPC is solvent. Assuming SIPC is solvent, that coverage would likely cover many of the IB customer accounts. For IB customer accounts in excess of $500,000, then those customers would have to claim against the Lloyd's policy. Only if those claims exceeding $500,000 from all IB customers exceeded $150 million in the aggregate would anybody be out in the cold.
As Def has noted, IB has been very conservative with their margin accounts to stave off most potential problems. Moreover, IB is well capitalized, from looking at their financials, available on their website. Therefore, the prospect that any particular customer would lose part or all of their account (of course we're not talking about market actions) is highly remote.
Yet because there is some risk, however remote, if your account exceeds $500,000, I myself decided today to open a separate individual account with IB in addition to my joint account, so I'd have the SIPC $500,000 protection on each account.
Def, please correct me if I misstated anything.