Investments in Securities and Security-Related Receivables (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2013		Dec. 31, 2012
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	$ 837,433		$ 915,435
Total investments in securities, Net Fair Value Adjustments	(234,260)		(259,926)
Investments in securities and security-related receivables, at fair value	603,173		655,509
Total investments in securities, Weighted Average Coupon	4.10%	[1]	4.60%	[1]
Total investments in securities, Weighted Average Years to Maturity	21 years 9 months 18 days		21 years
Trading securities
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	632,391		648,960
Total investments in securities, Net Fair Value Adjustments	(145,402)		(163,757)
Investments in securities and security-related receivables, at fair value	486,989		485,203
Total investments in securities, Weighted Average Coupon	3.80%	[1]	4.20%	[1]
Total investments in securities, Weighted Average Years to Maturity	21 years 3 months 18 days		21 years 2 months 12 days
Trading securities | TruPS and subordinated debentures
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	620,376		637,376
Total investments in securities, Net Fair Value Adjustments	(133,387)		(152,173)
Investments in securities and security-related receivables, at fair value	486,989		485,203
Total investments in securities, Weighted Average Coupon	3.80%	[1]	4.20%	[1]
Total investments in securities, Weighted Average Years to Maturity	20 years 10 months 24 days		20 years 10 months 24 days
Trading securities | Other securities
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	12,015		11,584
Total investments in securities, Net Fair Value Adjustments	(12,015)		(11,584)
Investments in securities and security-related receivables, at fair value	0		0
Total investments in securities, Weighted Average Coupon	4.80%	[1]	4.80%	[1]
Total investments in securities, Weighted Average Years to Maturity	39 years 4 months 24 days		39 years 10 months 24 days
Available-for-sale Securities
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	3,600		3,600
Total investments in securities, Net Fair Value Adjustments	(3,598)		(3,598)
Investments in securities and security-related receivables, at fair value	2		2
Total investments in securities, Weighted Average Coupon	2.10%	[1]	2.10%	[1]
Total investments in securities, Weighted Average Years to Maturity	29 years 4 months 24 days		29 years 10 months 24 days
Security-related receivables
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	201,442		262,875
Total investments in securities, Net Fair Value Adjustments	(85,260)		(92,571)
Investments in securities and security-related receivables, at fair value	116,182		170,304
Total investments in securities, Weighted Average Coupon	5.20%	[1]	5.70%	[1]
Total investments in securities, Weighted Average Years to Maturity	23 years		20 years
Security-related receivables | Trups Receivables
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	57,900		111,025
Total investments in securities, Net Fair Value Adjustments	(21,318)		(19,877)
Investments in securities and security-related receivables, at fair value	36,582		91,148
Total investments in securities, Weighted Average Coupon	5.40%	[1]	6.50%	[1]
Total investments in securities, Weighted Average Years to Maturity	13 years		9 years 1 month 6 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, Net Fair Value Adjustments	2,564		2,769
Investments in securities and security-related receivables, at fair value	32,564		32,769
Total investments in securities, Weighted Average Coupon	6.70%	[1]	6.70%	[1]
Total investments in securities, Weighted Average Years to Maturity	3 years 7 months 6 days		4 years 1 month 6 days
Security-related receivables | CMBS receivables
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	76,741	[2]	83,342	[3]
Total investments in securities, Net Fair Value Adjustments	(32,192)	[2]	(39,532)	[3]
Investments in securities and security-related receivables, at fair value	44,549	[2]	43,810	[3]
Total investments in securities, Weighted Average Coupon	5.60%	[1],[2]	5.60%	[1],[3]
Total investments in securities, Weighted Average Years to Maturity	31 years 3 months 18 days	[2]	32 years 2 months 12 days	[3]
Security-related receivables | Other securities
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	36,801		38,508
Total investments in securities, Net Fair Value Adjustments	(34,314)		(35,931)
Investments in securities and security-related receivables, at fair value	$ 2,487		$ 2,577
Total investments in securities, Weighted Average Coupon	2.60%	[1]	2.80%	[1]
Total investments in securities, Weighted Average Years to Maturity	37 years		37 years 4 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 $8,176 that are rated between "AAA" and "A-" by Standard & Poor's, securities with a fair value totaling $25,992 that are rated "BBB+" and "B-" by Standard & Poor's, securities with a fair value totaling $9,667 that are rated "CCC" by Standard & Poor's and securities with a fair value totaling $714 that are rated "D" by Standard & Poor's.

[3] CMBS receivables include securities with a fair value totaling $8,398 that are rated between "AAA" and "A-" by Standard & Poor's, securities with a fair value totaling $26,013 that are rated between "BBB+" and "B-" by Standard & Poor's, securities with a fair value totaling $8,782 that are rated "CCC" by Standard & Poor's, and securities with a fair value totaling $617 that are rated "D" by Standard & Poor's.