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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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