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Significant Accounting Policies (Schedule Of Securities Segregated For Regulatory Purposes) (Details) - USD ($) $ in Millions | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Marketable Securities [Line Items] |  |  |
Securities - segregated for regulatory purposes |  | $ 30,187 | $ 17,824
US Government Securities [Member] |  |  |
Marketable Securities [Line Items] |  |  |
Securities - segregated for regulatory purposes |  | 3,200 | 3,800
Securities Purchased Under Agreement To Resell [Member] |  |  |
Marketable Securities [Line Items] |  |  |
Securities - segregated for regulatory purposes | [1] | $ 27,000 | $ 14,000
[1] | These balances are collateralized by U.S. government securities.

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