Indebtedness - Additional Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended			0 Months Ended		9 Months Ended				1 Months Ended		0 Months Ended							0 Months Ended							9 Months Ended		12 Months Ended		9 Months Ended	1 Months Ended	9 Months Ended			9 Months Ended	12 Months Ended			0 Months Ended			9 Months Ended			9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended	0 Months Ended			0 Months Ended		9 Months Ended		9 Months Ended				9 Months Ended					9 Months Ended				9 Months Ended						9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended	0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Apr. 29, 2014 London Interest Bank Offer Rate	Sep. 30, 2014 Second Capped Call Option	Sep. 30, 2014 Second Capped Call Option Minimum	Sep. 30, 2014 Second Capped Call Option Maximum	Sep. 30, 2014 4.0% convertible senior notes	Dec. 31, 2013 4.0% convertible senior notes	Jan. 31, 2014 4.0% convertible senior notes Overallotment Option	Jan. 31, 2014 4.0% convertible senior notes Additional Principal Amount Overallotment Option Exercised By Underwriters	Apr. 14, 2014 7.625% senior notes due 2024	Apr. 14, 2014 7.625% senior notes due 2024	Apr. 29, 2014 CMBS securitizations	Sep. 30, 2014 CMBS Facilities	Dec. 31, 2013 CMBS Facilities	Sep. 30, 2014 7.0% convertible senior notes	Dec. 31, 2013 7.0% convertible senior notes	Aug. 11, 2014 7.125% Senior Notes due 2019	Aug. 11, 2014 7.125% Senior Notes due 2019	Sep. 30, 2014 Recourse Indebtedness		Jul. 28, 2014 Recourse Indebtedness	Dec. 31, 2013 Recourse Indebtedness		Sep. 30, 2014 Recourse Indebtedness 4.0% convertible senior notes		Dec. 31, 2013 Recourse Indebtedness 4.0% convertible senior notes		Sep. 30, 2014 Recourse Indebtedness 7.625% senior notes due 2024	Jan. 31, 2014 Recourse Indebtedness Secured credit facilities	Sep. 30, 2014 Recourse Indebtedness Secured credit facilities	Sep. 09, 2014 Recourse Indebtedness Secured credit facilities	Dec. 31, 2013 Recourse Indebtedness Secured credit facilities	Sep. 30, 2014 Recourse Indebtedness Secured credit facilities London Interest Bank Offer Rate	Dec. 31, 2013 Recourse Indebtedness Secured credit facilities Minimum	Dec. 31, 2013 Recourse Indebtedness Secured credit facilities Maximum	Sep. 09, 2014 Recourse Indebtedness Amended IROP Credit agreement	Sep. 09, 2014 Recourse Indebtedness Amended IROP Credit agreement London Interest Bank Offer Rate	Jan. 27, 2014 Recourse Indebtedness CMBS securitizations $75,000 Thousand Commercial Mortgage Facility	Jan. 27, 2014 Recourse Indebtedness CMBS securitizations $75,000 Thousand Commercial Mortgage Facility	Sep. 30, 2014 Recourse Indebtedness CMBS securitizations $75,000 Thousand Commercial Mortgage Facility London Interest Bank Offer Rate	Sep. 30, 2014 Recourse Indebtedness CMBS Facilities	Dec. 31, 2013 Recourse Indebtedness CMBS Facilities	Sep. 30, 2014 Recourse Indebtedness CMBS Facilities Minimum	Dec. 31, 2013 Recourse Indebtedness CMBS Facilities Minimum	Sep. 30, 2014 Recourse Indebtedness CMBS Facilities Maximum	Dec. 31, 2013 Recourse Indebtedness CMBS Facilities Maximum	Sep. 30, 2014 Recourse Indebtedness CMBS Facilities $250,000 Thousand CMBS facility	Sep. 30, 2014 Recourse Indebtedness 7.0% convertible senior notes		Dec. 31, 2013 Recourse Indebtedness 7.0% convertible senior notes		Sep. 30, 2014 Recourse Indebtedness 7.125% Senior Notes due 2019	Jul. 28, 2014 Recourse Indebtedness Amended Master Repurchase Agreement	Jul. 28, 2014 Recourse Indebtedness Amended Master Repurchase Agreement	Jul. 28, 2014 Recourse Indebtedness Amended Master Repurchase Agreement Floating Rate Loans	Jul. 28, 2014 Recourse Indebtedness Amended Master Repurchase Agreement Floating Rate Loans London Interest Bank Offer Rate	Jul. 28, 2014 Recourse Indebtedness Amended Master Repurchase Agreement Fixed Rate Loans London Interest Bank Offer Rate	Sep. 30, 2014 Investment Grade Senior Notes	Apr. 29, 2014 Investment Grade Senior Notes	Sep. 30, 2014 Investment Grade Senior Notes Upon Occurrence Of Certain Events	Apr. 29, 2014 Equity interest	Sep. 30, 2014 Non-recourse indebtedness	Dec. 31, 2013 Non-recourse indebtedness	Sep. 30, 2014 Non-recourse indebtedness CMBS securitizations	Sep. 30, 2014 Non-recourse indebtedness CDO notes payable, at amortized cost		Dec. 31, 2013 Non-recourse indebtedness CDO notes payable, at amortized cost		Sep. 30, 2014 Non-recourse indebtedness CDO notes payable, at fair value		Dec. 31, 2013 Non-recourse indebtedness CDO notes payable, at fair value		Sep. 30, 2014 Non-recourse indebtedness CDO notes payable, at fair value re-directed from retained interests	Sep. 30, 2014 Non-recourse indebtedness CDO notes payable, at fair value Principal collections of the underlying collateral	Sep. 30, 2014 Non-recourse indebtedness Loans payable on real estate		Dec. 31, 2013 Non-recourse indebtedness Loans payable on real estate		Sep. 30, 2014 Non-recourse indebtedness Loans payable on real estate Minimum		Dec. 31, 2013 Non-recourse indebtedness Loans payable on real estate Minimum		Sep. 30, 2014 Non-recourse indebtedness Loans payable on real estate Maximum		Dec. 31, 2013 Non-recourse indebtedness Loans payable on real estate Maximum		Sep. 30, 2014 Non-recourse indebtedness Loans payable on real estate First Mortgage MortgageLoan	Nov. 04, 2014 Non-recourse indebtedness Loans payable on real estate First Mortgage Subsequent Event	Nov. 04, 2014 Non-recourse indebtedness Loans payable on real estate First Mortgage Subsequent Event MortgageLoan
Debt Instrument [Line Items]
Convertible note conversion rate																										105.8429																								156.8716
Principal amount of convertible note, conversion rate																										$ 1																								$ 1
Initial conversion price																										$ 9.45																								$ 6.37
Principal amount of convertible senior notes issued	80,000		86,000							16,750	141,750		60,000							71,905
Proceeds from issuance of convertible senior notes										16,300	137,238
Discount on convertible senior notes																										1,182
Cap price of capped call transaction					11.91
Initial strike price of capped call transaction					$ 9.57
Capped call expiration date						2018-06	2018-10
Stated interest rate	4.00%							4.00%	4.00%				7.625%				7.00%	7.00%		7.125%																																																																							3.70%
Net proceeds from notes issued												57,500							69,209
Senior secured notes, maturity date												Apr. 15, 2024							Aug. 30, 2019
Redemption price percentage												100.00%							100.00%
Repurchase price percentage												101.00%							101.00%
Redemption period, Start date												Apr. 15, 2017							Aug. 30, 2017
Revolving credit agreement term																																3 years
Outstanding borrowings																																5,000																	159,171
LIBOR plus interest rate				1.79%																															2.50%				2.50%			2.00%																2.00%	2.50%
Line of credit facility, borrowing capacity																							100,000										20,000					30,000			75,000								575,000							200,000	100,000
Payment of notes	25,589	9,268																													6,143																																			40,482					18,255				4,125	14,130
Line of credit facility, remaining borrowing capacity																																																	415,829
Line of credit facility, expiration date																																																							Jul. 28, 2016
Credit agreement term																																								2 years
Principal amount repurchase convertible senior notes																																																																			5,800
Purchase price of notes																																																																			3,379
Gain on extinguishment of debt	2,421																																																																		2,421
Real estate investment	5,500													196,052																																															155,861		40,191
Real estate investment advance rate																																																													79.50%
Maturity date of investment																																																												2031-05		2016-04
Carrying Amount	2,613,317		2,086,401												1,573,559	1,662,537					501,273	[1]		235,011	[2]	133,752	[3]	116,184	[3]	60,000		5,000		11,129									159,171	37,749						33,297	[4]	32,938	[4]	71,905										2,112,044	1,851,390		1,084,289	[5],[6]	1,202,772	[5],[7]	431,142	[5],[8],[9]	377,235	[10],[5],[8]			381,095	[11]	171,244	[12]
Number of mortgages obtained																																																																																									11		1
Unpaid Principal Balance	$ 3,043,419		$ 2,592,414																		$ 515,663	[1]		$ 251,715	[2]	$ 141,750	[3]	$ 125,000	[3]	$ 60,000		$ 5,000		$ 11,129									$ 159,171	$ 37,746						$ 34,066	[4]	$ 34,066	[4]	$ 71,905										$ 2,527,756	$ 2,340,699		$ 1,086,038	[5],[6]	$ 1,204,117	[5],[7]	$ 846,941	[5],[8],[9]	$ 865,199	[10],[5],[8]			$ 379,259	[11]	$ 171,244	[12]									$ 210,483		$ 15,991
Contractual Maturity, start year																																																																																									2016-04
Contractual Maturity, end year																																																																																									2024-08
Stated interest rate, minimum																																																																																									3.40%
Stated interest rate, maximum																																																																																									5.60%
Contractual Maturity, month and year																										2033-10	[3]	2033-10	[3]	2024-04		2016-10				2016-10	2016-12								2014-11	2014-11	2016-07	2015-10		2031-04	[4]	2031-04	[4]	2019-08																											2015-09	[11]	2015-09	[12]	2024-08	[11]	2023-12	[12]		2021-11

[1] Excludes senior secured notes issued by us with an aggregate principal amount equal to $80,000 with a weighted average coupon of 7.0%, which are eliminated in consolidation.

[2] Excludes senior secured notes issued by us with an aggregate principal amount equal to $86,000 with a weighted average coupon of 7.0%, which are eliminated in consolidation.

[3] Our 4.0% convertible senior notes are redeemable at par, at the option of the holder, in October 2018, October 2023, and October 2028.

[4] Our 7.0% convertible senior notes are redeemable at par, at the option of the holder, in April 2016, April 2021, and April 2026.

[5] Excludes CDO notes payable purchased by us which are eliminated in consolidation.

[6] Collateralized by $1,573,559 principal amount of commercial mortgages, mezzanine loans, other loans and preferred equity interests. These obligations were issued by separate legal entities and consequently the assets of the special purpose entities that collateralize these obligations are not available to our creditors.

[7] Collateralized by $1,662,537 principal amount of commercial mortgages, mezzanine loans, other loans and preferred equity interests. These obligations were issued by separate legal entities and consequently the assets of the special purpose entities that collateralize these obligations are not available to our creditors.

[8] Relates to liabilities which we elected to record at fair value under FASB ASC Topic 825.

[9] Collateralized by $959,310 principal amount of investments in securities and security-related receivables and loans, before fair value adjustments. The fair value of these investments as of September 30, 2014 was $731,911. These obligations were issued by separate legal entities and consequently the assets of the special purpose entities that collateralize these obligations are not available to our creditors.

[10] Collateralized by $989,781 principal amount of investments in securities and security-related receivables and loans, before fair value adjustments. The fair value of these investments as of December 31, 2013 was $746,939. These obligations were issued by separate legal entities and consequently the assets of the special purpose entities that collateralize these obligations are not available to our creditors.

[11] Includes $246,997 of unpaid principal balance with a carrying amount of $248,833 of mortgage indebtedness that encumbers properties owned by IRT. The weighted-average interest rate is 3.7% and has a range of maturity dates from April 2016 to August 2024.

[12] Includes $100,803 of unpaid principal balance and carrying amount of mortgage indebtedness that encumbers properties owned by IRT. The weighted-average interest rate is 3.8% and has a range of maturity dates from January 2019 to November 2022.