Fair Value Disclosures - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2017	Aug. 31, 2016	Nov. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments at fair value	$ 24,200		$ 24,200		$ 24,300
Transfers of assets from Level 2 to Level 3	63,500	$ 147,000	26,500	$ 157,800
Transfers of assets from Level 3 to Level 2	35,700	91,800	49,500	114,500
Transfers of liabilities from Level 3 to Level 2				10,200
Net gains/(losses) on Level 3 assets (realized and unrealized)	6,000	(48,300)	(900)	(93,100)
Net gains/(losses) on Level 3 liabilities (realized and unrealized)	6,000	5,400	1,500	(2,800)
Aggregate fair value of loans and other receivables on nonaccrual status and/or greater than 90 days past due	62,600		62,600		29,800
Loans and other receivables greater than 90 days past due	41,900		41,900		18,900
Cash and securities segregated and on deposit for regulatory purposes or deposited with clearing and depository organizations includes U.S. treasury securities	903,744		903,744		857,337
Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Cash and securities segregated and on deposit for regulatory purposes or deposited with clearing and depository organizations includes U.S. treasury securities	99,800		99,800		$ 99,900
Long-term debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other changes in fair value	(1,854)	4,474	2,786	15,225
CDOs and CLOs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		23,700	9,600	16,900
Transfers of assets from Level 3 to Level 2		27,300		25,300
Net gains/(losses) on Level 3 assets (realized and unrealized)	(6,119)	(4,067)	(13,902)	(24,356)
Municipal securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		34,300		28,700
Net gains/(losses) on Level 3 assets (realized and unrealized)		(7,074)	(1,547)	(1,462)
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		13,700		21,700
Transfers of assets from Level 3 to Level 2	14,600	18,300		25,500
Net gains/(losses) on Level 3 assets (realized and unrealized)	(263)	(2,343)	(1,446)	(7,243)
Investments at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		29,000		26,100
Net gains/(losses) on Level 3 assets (realized and unrealized)	1,703	(966)	4,698	(19,055)
Other ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	46,400	27,300		38,500
Transfers of assets from Level 3 to Level 2	13,500			17,000
Net gains/(losses) on Level 3 assets (realized and unrealized)	(6,454)	(2,247)	(15,338)	(2,420)
Corporate equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 3 to Level 2		20,500		19,200
Net gains/(losses) on Level 3 assets (realized and unrealized)	4,344	(6,492)	3,416	(8,388)
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3				17,200
Net gains/(losses) on Level 3 assets (realized and unrealized)	(125)	(1,531)	(1,180)	(4,606)
Loans and other receivables
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 3 to Level 2		15,800	28,300	19,800
Net gains/(losses) on Level 3 assets (realized and unrealized)	15,261	(23,445)	27,709	(30,843)
Corporate debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3			8,100
Net gains/(losses) on Level 3 assets (realized and unrealized)	$ (2,350)	$ (145)	$ (3,280)	$ 5,239