Summary of Commercial Mortgage Loans, Mezzanine Loans, Preferred Equity and Other loans by Geographic location (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Mortgage Loans On Real Estate [Line Items]
Carrying Amount of Mortgage Loans	$ 1,294,066	[1]	$ 1,625,838	$ 1,393,132
Equity Investments | Investment 1
Mortgage Loans On Real Estate [Line Items]
Carrying Amount of Mortgage Loans	3,700
Equity Investments | Investment 2
Mortgage Loans On Real Estate [Line Items]
Carrying Amount of Mortgage Loans	$ 2,500

[1] The tax basis of the commercial mortgage loans, mezzanine loans and preferred equity interests approximates the carrying amount.