Collateralized Securities Transactions (Narrative) (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) item	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) item	Apr. 25, 2018 USD ($)
Concentration Risk [Line Items]
Number of major groups of repo business | item	2
Securities reverse repurchase agreements	$ 6,014,438,000		$ 7,500,002,000
Fair value of securities received as collateral under reverse repurchase agreements	$ 6,167,233,000		$ 7,769,693,000
Number of counterparties related to reverse repurchase agreements | item	30		41
Securities sold under agreements to repurchase	$ 6,066,372,000		$ 7,534,443,000
Fair value of securities pledged as collateral under repurchase agreements	6,035,939,000		7,561,978,000
Intraday lending facility, commitment amount	$ 75,000,000		32,820,000
Intraday lending facility, termination date	Oct. 16, 2020
Intraday lending facility, renewal period	364 days
Intraday lending facility, annual interest rate	0.12%
Advances under intraday lending facility	$ 0		32,818,000
Intraday lending facility, accrued interest			$ 2,000
Revenues	$ 17,770,000	$ 11,140,000
Gestational Repo Business [Member]
Concentration Risk [Line Items]
Line of credit, borrowing capacity				$ 25,000,000
Number of counterparties related to reverse repurchase agreements | item	7		7
Base Rate [Member]
Concentration Risk [Line Items]
Intraday lending facility, basis spread on variable rate	3.00%
Feds Fund Rate [Member]
Concentration Risk [Line Items]
Intraday lending facility, basis spread on variable rate	0.50%
FICC [Member]
Concentration Risk [Line Items]
Securities reverse repurchase agreements	$ 1,473,033,000		$ 371,025,000
Securities sold under agreements to repurchase	4,570,581,000		$ 5,138,712,000
Product Concentration Risk [Member]
Concentration Risk [Line Items]
Revenues	$ 6,868,000	$ 1,294,000