Variable Interest Entities (Narrative) (Details) - USD ($) $ in Thousands	May 31, 2019	Nov. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Carrying Amount	$ 1,080,400	$ 746,400
Investment in associated company	2,415,894	2,417,332	$ 2,522,944	$ 2,066,829
Other investment vehicles
Variable Interest Entity [Line Items]
Unfunded equity commitment related to investments	19,800	11,100
Carrying Amount	507,000	203,600
Agency Mortgage-Backed Securitizations
Variable Interest Entity [Line Items]
Carrying Amount	1,525,100	2,913,000
Nonagency Mortgage- And Asset-Backed Securitizations
Variable Interest Entity [Line Items]
Carrying Amount	34,700	170,500
JCP Entities | Private Equity Vehicles
Variable Interest Entity [Line Items]
Equity commitments	139,300	139,300
Funded equity commitments	121,500	121,300
Carrying amount of equity investment	31,200	$ 35,500
FXCM
Variable Interest Entity [Line Items]
Fair value of senior secured term loan receivable	56,600
Investment in associated company	70,000
Senior secured term loan receivable and investments in associated companies	$ 126,600