Non-Accrual Status of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Investment Holdings [Line Items]
Weighted Average Coupon	4.00%	[1]	3.90%	[1]
TruPS and TruPS receivables
Investment Holdings [Line Items]
Principal/Par Amount on Non Accrual	$ 65,557		$ 83,557
Weighted Average Coupon	2.00%		1.80%
Fair Value	650		5,678
Other securities
Investment Holdings [Line Items]
Principal/Par Amount on Non Accrual	36,842		41,019
Weighted Average Coupon	3.20%		3.10%
Fair Value	630		11
CMBS receivables
Investment Holdings [Line Items]
Principal/Par Amount on Non Accrual	15,250		22,772
Weighted Average Coupon	5.90%		5.90%
Fair Value	$ 328		$ 447

[1] Weighted-average coupon is calculated on the unpaid principal amount of the underlying instruments, which does not necessarily correspond to the carrying amount.