Investments in Securities and Security-Related Receivables (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2014	Mar. 31, 2015
Investment Securities [Line Items]
Total investments in securities, Amortized Cost		$ 225,600
Investments in securities and security-related receivables, at fair value		$ 31,412	$ 31,412
Total investments in securities, Weighted Average Coupon	[1]	3.60%
Total investments in securities, Weighted Average Years to Maturity		22 years 8 months 12 days
Available-for-sale Securities
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	[2]	$ 210,600
Investments in securities and security-related receivables, at fair value	[2]	$ 19,167
Total investments in securities, Weighted Average Coupon	[1],[2]	3.50%
Total investments in securities, Weighted Average Years to Maturity	[2]	23 years 1 month 6 days
Security-related receivables
Investment Securities [Line Items]
Total investments in securities, Amortized Cost		$ 15,000
Investments in securities and security-related receivables, at fair value		$ 12,245
Total investments in securities, Weighted Average Coupon	[1]	6.30%
Total investments in securities, Weighted Average Years to Maturity		13 years 6 months
Security-related receivables | Unsecured REIT note receivables
Investment Securities [Line Items]
Total investments in securities, Amortized Cost		$ 10,000
Investments in securities and security-related receivables, at fair value		$ 10,995
Total investments in securities, Weighted Average Coupon	[1]	6.70%
Total investments in securities, Weighted Average Years to Maturity		3 years
Security-related receivables | CMBS receivables
Investment Securities [Line Items]
Total investments in securities, Amortized Cost	[3]	$ 5,000
Investments in securities and security-related receivables, at fair value	[3]	$ 1,250
Total investments in securities, Weighted Average Coupon	[1],[3]	5.70%
Total investments in securities, Weighted Average Years to Maturity	[3]	34 years 6 months

[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] As of December 31, 2014, this includes available-for-sale securities that are accounted for under the fair value option other than an available-for-sale security that has an amortized cost of $3,600 and a carrying value of $3,606.

[3] CMBS receivables include securities with a fair value totaling $1,250 that are rated “D” by Standard & Poor’s.