Fair Value Disclosures - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	May 31, 2017	May 31, 2016	May 31, 2017	May 31, 2016	Nov. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments at fair value	$ 23,700		$ 23,700		$ 24,300
Transfers of assets from Level 2 to Level 3	23,900	$ 107,100	36,600	$ 155,900
Transfers of assets from Level 3 to Level 2	23,800	62,700	54,500	92,900
Net gains/(losses) on Level 3 assets (realized and unrealized)	(2,700)	(21,400)	(12,000)	(51,900)
Net gains/(losses) on Level 3 liabilities (realized and unrealized)	(300)	(300)	2,400	(9,500)
Aggregate fair value of loans and other receivables on nonaccrual status and/or greater than 90 days past due	36,200		36,200		29,800
Loans and other receivables greater than 90 days past due	28,500		28,500		18,900
Cash and securities segregated and on deposit for regulatory purposes or deposited with clearing and depository organizations	919,011		919,011		857,337
Level 1 [Member]
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	99,800		99,800		$ 99,900
Long-term debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other changes in fair value	1,516	3,893	4,933	10,751
CDOs and CLOs [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3			12,400	30,600
Transfers of assets from Level 3 to Level 2				22,300
Net gains/(losses) on Level 3 assets (realized and unrealized)	(2,721)	1,797	(10,102)	(20,455)
Municipal securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net gains/(losses) on Level 3 assets (realized and unrealized)	(70)		(1,547)
RMBS [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3	12,000	19,300	11,500
Transfers of assets from Level 3 to Level 2	12,600
Net gains/(losses) on Level 3 assets (realized and unrealized)	(2,188)	(4,915)	(3,000)	(8,337)
Non-agency RMBS [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3				21,700
Transfers of assets from Level 3 to Level 2		19,500		16,700
Investments at fair value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3				26,100
Transfers of assets from Level 3 to Level 2				16,100
Net gains/(losses) on Level 3 assets (realized and unrealized)	5,194	(1,574)	2,995	(6,090)
Other ABS [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		30,700		28,000
Transfers of assets from Level 3 to Level 2		16,000
Net gains/(losses) on Level 3 assets (realized and unrealized)	(3,663)	(7,284)	(5,489)	(202)
Corporate equity securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		22,000
Net gains/(losses) on Level 3 assets (realized and unrealized)	(1,198)	(927)	(489)	1,571
CMBS [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net gains/(losses) on Level 3 assets (realized and unrealized)	98	(1,140)	(1,119)	(2,589)
Loans and other receivables [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers of assets from Level 2 to Level 3		15,900		20,200
Transfers of assets from Level 3 to Level 2			30,800	16,800
Net gains/(losses) on Level 3 assets (realized and unrealized)	3,226	(7,792)	10,062	(13,376)
Corporate debt securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net gains/(losses) on Level 3 assets (realized and unrealized)	$ (1,420)	$ 474	$ (3,300)	$ (2,378)