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Collateralized Securities Transactions (Narrative) (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) entity	Apr. 25, 2018 USD ($)
Concentration Risk [Line Items]
Securities reverse repurchase agreements	$ 4,464,324		$ 4,464,324		$ 1,680,883
Fair value of securities received as collateral under reverse repurchase agreements	4,654,319		4,654,319		$ 1,753,978
Number of counterparties related to reverse repurchase agreements | entity					19
Securities sold under agreements to repurchase	4,506,628		4,506,628		$ 1,692,279
Fair value of securities pledged as collateral under repurchase agreements	4,528,074		4,528,074		$ 1,708,154
Intraday lending facility, commitment amount	100,000		$ 100,000
Intraday lending facility, termination date			Oct. 19, 2019
Intraday lending facility, renewal period			minus 364 days
Intraday lending facility, annual interest rate			0.12%
Revenues	12,237	$ 10,001	$ 33,765	$ 35,867
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] | Minimum [Member]
Concentration Risk [Line Items]
Line of credit, borrowing capacity						$ 25,000
Product Concentration Risk [Member]
Concentration Risk [Line Items]
Revenues	$ 1,401	$ 1,091	$ 3,450	$ 2,633

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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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