Non-Accrual Status of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Investment Holdings [Line Items]
Weighted Average Coupon	4.10%	[1]	4.60%	[1]
TruPS and TruPS receivables
Investment Holdings [Line Items]
Principal/Par Amount on Non Accrual	$ 83,557		$ 83,557
Weighted Average Coupon	1.90%		1.90%
Fair Value	5,678		5,678
Other securities
Investment Holdings [Line Items]
Principal/Par Amount on Non Accrual	40,448		35,159
Weighted Average Coupon	3.00%		3.20%
Fair Value	2		2
CMBS receivables
Investment Holdings [Line Items]
Principal/Par Amount on Non Accrual	28,608		35,208
Weighted Average Coupon	5.90%		5.90%
Fair Value	$ 746		$ 642

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