Real Estate and Lending Activities - Schedule of Revenue from External Customers by Geographic Areas (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 156,397	$ 134,999
Revenue [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 156,397	$ 134,999
Percentage of Total Revenue	100.00%	100.00%
Revenue [Member] | Massachusetts [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 26,584
Percentage of Total Revenue	17.00%
Revenue [Member] | Texas [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 24,737	$ 24,472
Percentage of Total Revenue	15.80%	18.10%
Revenue [Member] | California [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 16,565	$ 16,597
Percentage of Total Revenue	10.60%	12.30%
Revenue [Member] | New Jersey [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 10,943	$ 8,612
Percentage of Total Revenue	7.00%	6.40%
Revenue [Member] | Arizona [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 7,332	$ 5,797
Percentage of Total Revenue	4.70%	4.30%
Revenue [Member] | All Other States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 43,056	$ 54,906
Percentage of Total Revenue	27.50%	40.70%
Revenue [Member] | United States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 129,217	$ 110,384
Percentage of Total Revenue	82.60%	81.80%
Revenue [Member] | Germany [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 26,190	$ 23,510
Percentage of Total Revenue	16.70%	17.40%
Revenue [Member] | United Kingdom, Italy, and Spain [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 990	$ 1,105
Percentage of Total Revenue	0.70%	0.80%
Revenue [Member] | International [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 27,180	$ 24,615
Percentage of Total Revenue	17.40%	18.20%