Fair Value Hierarchy for Financial Instruments Not Fair Valued but For Which Fair Value Is Required To Be Disclosed (Detail) - USD ($) $ in Thousands	Jun. 30, 2015		Dec. 31, 2014
Carrying Amount
Commercial mortgages, mezzanine loans and other loans, Carrying Amount	$ 1,506,542		$ 1,392,436
Carrying Amount	2,661,528		2,615,666
Estimated Fair Value
Junior subordinated notes	12,866		13,102
Estimated Fair Value
Estimated Fair Value
Commercial mortgages, mezzanine loans and other loans, Estimated Fair Value	1,448,755		1,316,796
Carrying Amount
Carrying Amount
Commercial mortgages, mezzanine loans and other loans, Carrying Amount	1,493,746		1,392,436
CMBS securitizations
Carrying Amount
Carrying Amount	676,161		490,863
CMBS securitizations | Estimated Fair Value
Estimated Fair Value
Notes payable, Estimated Fair Value	536,528		389,771
CMBS securitizations | Carrying Amount
Carrying Amount
Carrying Amount	536,796		389,415
7.0% convertible senior notes | Estimated Fair Value
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	36,882		41,901
7.0% convertible senior notes | Carrying Amount
Carrying Amount
Carrying Amount	33,658		33,417
4.0% convertible senior notes | Estimated Fair Value
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	120,480		123,677
4.0% convertible senior notes | Carrying Amount
Carrying Amount
Carrying Amount	135,752		134,418
7.625% senior notes due 2024 | Estimated Fair Value
Estimated Fair Value
Senior notes, Estimated Fair Value	55,440		56,016
7.625% senior notes due 2024 | Carrying Amount
Carrying Amount
Carrying Amount	60,000		60,000
7.125% Senior Notes due 2019 | Estimated Fair Value
Estimated Fair Value
Senior notes, Estimated Fair Value	71,330		70,064
7.125% Senior Notes due 2019 | Carrying Amount
Carrying Amount
Carrying Amount	71,905		71,905
Senior Secured Notes | Estimated Fair Value
Estimated Fair Value
Senior notes, Estimated Fair Value	75,777		79,594
Senior Secured Notes | Carrying Amount
Carrying Amount
Carrying Amount	65,559		68,314
Junior subordinated notes, at amortized cost | Estimated Fair Value
Estimated Fair Value
Junior subordinated notes	14,107		14,471
Junior subordinated notes, at amortized cost | Carrying Amount
Carrying Amount
Carrying Amount	25,100		25,100
CDO notes payable, at amortized cost | Estimated Fair Value
Estimated Fair Value
Notes payable, Estimated Fair Value	807,995		913,050
CDO notes payable, at amortized cost | Carrying Amount
Carrying Amount
Carrying Amount	986,891		1,073,145
Loans payable on real estate | Estimated Fair Value
Estimated Fair Value
Loans payable on real estate, Estimated Fair Value	667,791		678,306
Loans payable on real estate | Carrying Amount
Carrying Amount
Carrying Amount	640,411		643,405
Quoted Prices in Active Markets for Identical Assets (Level 1) | CMBS securitizations
Estimated Fair Value
Notes payable, Estimated Fair Value	536,528		389,771
Quoted Prices in Active Markets for Identical Assets (Level 1) | 7.0% convertible senior notes
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	36,882		41,901
Quoted Prices in Active Markets for Identical Assets (Level 1) | 4.0% convertible senior notes
Estimated Fair Value
Convertible senior notes, Estimated Fair Value	120,480		123,677
Quoted Prices in Active Markets for Identical Assets (Level 1) | 7.625% senior notes due 2024
Estimated Fair Value
Senior notes, Estimated Fair Value	55,440		56,016
Quoted Prices in Active Markets for Identical Assets (Level 1) | 7.125% Senior Notes due 2019
Estimated Fair Value
Senior notes, Estimated Fair Value	71,330		70,064
Quoted Prices in Active Markets for Identical Assets (Level 1) | CDO notes payable, at amortized cost
Estimated Fair Value
Notes payable, Estimated Fair Value	807,995		913,050
Significant Unobservable Inputs (Level 3)
Estimated Fair Value
Commercial mortgages, mezzanine loans and other loans, Estimated Fair Value	1,448,755		1,316,796
Junior subordinated notes	12,866	[1]	13,102	[2]
Significant Unobservable Inputs (Level 3) | Senior Secured Notes
Estimated Fair Value
Senior notes, Estimated Fair Value	75,777		79,594
Significant Unobservable Inputs (Level 3) | Junior subordinated notes, at amortized cost
Estimated Fair Value
Junior subordinated notes	14,107		14,471
Significant Unobservable Inputs (Level 3) | Loans payable on real estate
Estimated Fair Value
Loans payable on real estate, Estimated Fair Value	$ 667,791		$ 678,306

[1] During the six-month period ended June 30, 2015, there were no transfers between Level 1 and Level 2, nor were there any transfers into and out of Level 3.

[2] During the year ended December 31, 2014, there were no transfers between Level 1 and Level 2, nor were there any transfers into and out of Level 3.