Commitments and Contingencies - Collateral Available, Loaned or Repledged (Details) - USD ($) $ in Billions	Dec. 31, 2019	Sep. 30, 2019
Customer Securities for which Entity has Right to Sell or Repledge (Including Securities Sold or Repledged) [Line Items]
Total collateral available	$ 30.3	$ 30.5
Collateral loaned	2.4	3.2
Collateral repledged	4.5	4.6
Total collateral loaned or repledged	6.9	7.8
Client Margin Securities [Member]
Customer Securities for which Entity has Right to Sell or Repledge (Including Securities Sold or Repledged) [Line Items]
Total collateral available	29.1	28.6
Stock Borrowings [Member]
Customer Securities for which Entity has Right to Sell or Repledge (Including Securities Sold or Repledged) [Line Items]
Total collateral available	$ 1.2	$ 1.9