Investments in Loans (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014 Loan		Dec. 31, 2013 Loan
Mortgage Loans on Real Estate [Line Items]
Number of Loans	174		153
Carrying Amount	$ 1,370,966		$ 1,123,177
Weighted-Average Coupon	6.60%	[1]	7.20%	[1]
Commercial mortgages
Mortgage Loans on Real Estate [Line Items]
Number of Loans	86	[2]	59	[3]
Carrying Amount	1,053,190	[2]	773,269	[3]
Weighted-Average Coupon	5.90%	[1],[2]	6.40%	[1],[3]
Range of Maturity Dates, Minimum	Dec. 01, 2014	[2]	Mar. 01, 2014	[3]
Range of Maturity Dates, Maximum	Jul. 01, 2044	[2]	Jan. 01, 2029	[3]
Conduit Loans Held for Sale | Commercial mortgages
Mortgage Loans on Real Estate [Line Items]
Number of Loans	11		6
Carrying Amount	$ 107,164		$ 61,825
Weighted-Average Coupon	4.60%		5.30%
Range of Maturity Dates, Minimum	Sep. 01, 2021		Jan. 01, 2024
Range of Maturity Dates, Maximum	Jul. 01, 2044		Jan. 01, 2029

[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] Commercial mortgages includes 11 conduit loans with an unpaid principal balance and carrying amount of $107,164, a weighted-average coupon of 4.6% and maturity dates ranging from September 2021 through July 2044. These commercial mortgages are classified as loans held for sale.

[3] Commercial mortgages includes six conduit loans with an unpaid principal balance and carrying amount of $61,825, a weighted-average coupon of 5.3% and maturity dates ranging from January 2024 through January 2029. These commercial mortgages are classified as loans held for sale.