Investments in Securities and Security-Related Receivables (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	$ 763,191		$ 802,237
Total investments in securities, Gains	4,188		4,768
Total investments in securities, Losses	(199,100)		(239,703)
Investments in securities and security-related receivables, at fair value	568,279		567,302
Total investments in securities, Weighted Average Coupon	4.00%	[1]	3.90%	[1]
Total investments in securities, Weighted Average Years to Maturity	21 years 2 months 12 days		21 years 8 months 12 days
Trading securities
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	614,980		632,688
Total investments in securities, Gains	0		0
Total investments in securities, Losses	(125,042)		(151,843)
Investments in securities and security-related receivables, at fair value	489,938		480,845
Total investments in securities, Weighted Average Coupon	3.80%	[1]	3.80%	[1]
Total investments in securities, Weighted Average Years to Maturity	20 years 6 months		20 years 9 months 18 days
Trading securities | TruPS and subordinated debentures
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	602,376		620,376
Total investments in securities, Gains	0		0
Total investments in securities, Losses	(112,438)		(139,531)
Investments in securities and security-related receivables, at fair value	489,938		480,845
Total investments in securities, Weighted Average Coupon	3.80%	[1]	3.70%	[1]
Total investments in securities, Weighted Average Years to Maturity	20 years 2 months 12 days		20 years 4 months 24 days
Trading securities | Other securities
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	12,604		12,312
Total investments in securities, Gains	0		0
Total investments in securities, Losses	(12,604)		(12,312)
Investments in securities and security-related receivables, at fair value	0		0
Total investments in securities, Weighted Average Coupon	4.70%	[1]	4.70%	[1]
Total investments in securities, Weighted Average Years to Maturity	38 years 1 month 6 days		38 years 10 months 24 days
Security-related receivables
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	144,611		165,949
Total investments in securities, Gains	4,188		4,768
Total investments in securities, Losses	(71,088)		(84,262)
Investments in securities and security-related receivables, at fair value	77,711		86,455
Total investments in securities, Weighted Average Coupon	4.70%	[1]	4.70%	[1]
Total investments in securities, Weighted Average Years to Maturity	23 years 3 months 18 days		24 years 4 months 24 days
Security-related receivables | Trups Receivables
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	32,900		32,900
Total investments in securities, Gains	0		0
Total investments in securities, Losses	(25,217)		(24,689)
Investments in securities and security-related receivables, at fair value	7,683		8,211
Total investments in securities, Weighted Average Coupon	3.40%	[1]	3.40%	[1]
Total investments in securities, Weighted Average Years to Maturity	17 years 6 months		18 years 3 months 18 days
Security-related receivables | Unsecured REIT note receivables
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	30,000		30,000
Total investments in securities, Gains	2,532		3,046
Total investments in securities, Losses	0		0
Investments in securities and security-related receivables, at fair value	32,532		33,046
Total investments in securities, Weighted Average Coupon	6.70%	[1]	6.70%	[1]
Total investments in securities, Weighted Average Years to Maturity	2 years 4 months 24 days		3 years 1 month 6 days
Security-related receivables | CMBS receivables
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	53,383	[2]	69,905	[3]
Total investments in securities, Gains	1,656	[2]	1,722	[3]
Total investments in securities, Losses	(18,604)	[2]	(27,509)	[3]
Investments in securities and security-related receivables, at fair value	36,435	[2]	44,118	[3]
Total investments in securities, Weighted Average Coupon	5.80%	[1],[2]	5.60%	[1],[3]
Total investments in securities, Weighted Average Years to Maturity	32 years	[2]	30 years 7 months 6 days	[3]
Security-related receivables | Other securities
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	28,328		33,144
Total investments in securities, Gains	0		0
Total investments in securities, Losses	(27,267)		(32,064)
Investments in securities and security-related receivables, at fair value	1,061		1,080
Total investments in securities, Weighted Average Coupon	2.30%	[1]	2.20%	[1]
Total investments in securities, Weighted Average Years to Maturity	35 years 7 months 6 days		36 years 6 months
Available-for-sale Securities
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	3,600		3,600
Total investments in securities, Gains	0		0
Total investments in securities, Losses	(2,970)		(3,598)
Investments in securities and security-related receivables, at fair value	$ 630		$ 2
Total investments in securities, Weighted Average Coupon	2.00%	[1]	2.00%	[1]
Total investments in securities, Weighted Average Years to Maturity	28 years 1 month 6 days		28 years 10 months 24 days

[1] Weighted-average coupon is calculated on the unpaid principal amount of the underlying instruments, which does not necessarily correspond to the carrying amount.

[2] CMBS receivables include securities with a fair value totaling $12,122 that are rated between "AAA" and "A-" by Standard & Poor's, securities with a fair value totaling $22,667 that are rated "BBB+" and "B-" by Standard & Poor's and securities with a fair value totaling $1,646 that are rated "D" by Standard & Poor's.

[3] CMBS receivables include securities with a fair value totaling $8,228 that are rated between "AAA" and "A-" by Standard & Poor's, securities with a fair value totaling $26,594 that are rated between "BBB+" and "B-" by Standard & Poor's, securities with a fair value totaling $8,164 that are rated "CCC" by Standard & Poor's, and securities with a fair value totaling $1,132 that are rated "D" by Standard & Poor's.