Information of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Assets:
Marketable Securities	$ 670,245		$ 655,509
Derivative assets	1,158		1,132
Total assets	671,403		656,641
Liabilities:
Junior subordinated notes	11,268		29,655
CDO notes payable, at fair value	282,355		187,048
Derivative liabilities	141,518		151,438
Total liabilities	435,141		368,141
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Derivative assets	0	[1]	0
Total assets	0	[1]	0
Liabilities:
Junior subordinated notes	0	[1]	0
CDO notes payable, at fair value	0	[1]	0
Derivative liabilities	0	[1]	0
Total liabilities	0	[1]	0
Significant Other Observable Inputs (Level 2)
Assets:
Derivative assets	1,158	[1]	1,132
Total assets	81,894	[1]	80,290
Liabilities:
Junior subordinated notes	0	[1]	0
CDO notes payable, at fair value	0	[1]	0
Derivative liabilities	65,405	[1]	71,976
Total liabilities	65,405	[1]	71,976
Significant Unobservable Inputs (Level 3)
Assets:
Derivative assets	0	[1]	0
Total assets	589,509	[1]	576,351
Liabilities:
Junior subordinated notes	11,268	[1]	29,655
CDO notes payable, at fair value	282,355	[1]	187,048
Derivative liabilities	76,113	[1]	79,462
Total liabilities	369,736	[1]	296,165
Trading securities
Assets:
Marketable Securities	498,287		485,203
Trading securities | Trups
Assets:
Marketable Securities	498,287		485,203
Trading securities | Other securities
Assets:
Marketable Securities	0		0
Trading securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Trups
Assets:
Marketable Securities	0	[1]	0
Trading securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other securities
Assets:
Marketable Securities	0	[1]	0
Trading securities | Significant Other Observable Inputs (Level 2) | Trups
Assets:
Marketable Securities	0	[1]	0
Trading securities | Significant Other Observable Inputs (Level 2) | Other securities
Assets:
Marketable Securities	0	[1]	0
Trading securities | Significant Unobservable Inputs (Level 3) | Trups
Assets:
Marketable Securities	498,287	[1]	485,203
Trading securities | Significant Unobservable Inputs (Level 3) | Other securities
Assets:
Marketable Securities	0	[1]	0
Available-for-sale Securities
Assets:
Marketable Securities	2		2
Available-for-sale Securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Marketable Securities	0	[1]	0
Available-for-sale Securities | Significant Other Observable Inputs (Level 2)
Assets:
Marketable Securities	2	[1]	2
Available-for-sale Securities | Significant Unobservable Inputs (Level 3)
Assets:
Marketable Securities	0	[1]	0
Security-related receivables
Assets:
Marketable Securities	171,956		170,304
Security-related receivables | Trups Receivables
Assets:
Marketable Securities	91,222		91,148
Security-related receivables | Unsecured REIT note receivables
Assets:
Marketable Securities	33,487		32,769
Security-related receivables | CMBS receivables
Assets:
Marketable Securities	44,852	[2]	43,810	[3]
Security-related receivables | Other securities
Assets:
Marketable Securities	2,395		2,577
Security-related receivables | Quoted Prices in Active Markets for Identical Assets (Level 1) | Trups Receivables
Assets:
Marketable Securities	0	[1]	0
Security-related receivables | Quoted Prices in Active Markets for Identical Assets (Level 1) | Unsecured REIT note receivables
Assets:
Marketable Securities	0	[1]	0
Security-related receivables | Quoted Prices in Active Markets for Identical Assets (Level 1) | CMBS receivables
Assets:
Marketable Securities	0	[1]	0
Security-related receivables | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other securities
Assets:
Marketable Securities	0	[1]	0
Security-related receivables | Significant Other Observable Inputs (Level 2) | Trups Receivables
Assets:
Marketable Securities	0	[1]	0
Security-related receivables | Significant Other Observable Inputs (Level 2) | Unsecured REIT note receivables
Assets:
Marketable Securities	33,487	[1]	32,769
Security-related receivables | Significant Other Observable Inputs (Level 2) | CMBS receivables
Assets:
Marketable Securities	44,852	[1]	43,810
Security-related receivables | Significant Other Observable Inputs (Level 2) | Other securities
Assets:
Marketable Securities	2,395	[1]	2,577
Security-related receivables | Significant Unobservable Inputs (Level 3) | Trups Receivables
Assets:
Marketable Securities	91,222	[1]	91,148
Security-related receivables | Significant Unobservable Inputs (Level 3) | Unsecured REIT note receivables
Assets:
Marketable Securities	0	[1]	0
Security-related receivables | Significant Unobservable Inputs (Level 3) | CMBS receivables
Assets:
Marketable Securities	0	[1]	0
Security-related receivables | Significant Unobservable Inputs (Level 3) | Other securities
Assets:
Marketable Securities	$ 0	[1]	$ 0

[1] During the three-month period ended March 31, 2013, there were no transfers between Level 1 and Level 2, as well as, there were no transfers into and out of Level 3.

[2] CMBS receivables include securities with a fair value totaling $8,287 that are rated between "AAA" and "A-" by Standard & Poor's, securities with a fair value totaling $26,972 that are rated "BBB+" and "B-" by Standard & Poor's, securities with a fair value totaling $9,057 that are rated "CCC" by Standard & Poor's and securities with a fair value totaling $536 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.