Real Estate and Lending Activities - Schedule of Revenue from External Customers by Geographic Areas (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 176,580	$ 126,555	$ 499,784	$ 387,854
Revenue [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 499,784	$ 387,854
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			$ 79,741
Percentage of Total Revenue			16.00%
Revenue [Member] | Texas [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 74,489	$ 72,811
Percentage of Total Revenue			14.90%	18.80%
Revenue [Member] | California [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 49,681	$ 49,724
Percentage of Total Revenue			9.90%	12.80%
Revenue [Member] | New Jersey [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 32,756	$ 28,398
Percentage of Total Revenue			6.60%	7.30%
Revenue [Member] | Arizona [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 23,902	$ 17,678
Percentage of Total Revenue			4.80%	4.60%
Revenue [Member] | All Other States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 147,606	$ 143,289
Percentage of Total Revenue			29.50%	36.90%
Revenue [Member] | United States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 408,175	$ 311,900
Percentage of Total Revenue			81.70%	80.40%
Revenue [Member] | Germany [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 88,525	$ 72,718
Percentage of Total Revenue			17.70%	18.80%
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			$ 3,084	$ 3,236
Percentage of Total Revenue			0.60%	0.80%
Revenue [Member] | International [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 91,609	$ 75,954
Percentage of Total Revenue			18.30%	19.60%