Real Estate and Lending Activities - Schedule of Revenue from External Customers by Geographic Areas (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 205,046	$ 156,397
Revenue [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 205,046	$ 156,397
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	$ 26,940	$ 26,584
Percentage of Total Revenue	13.10%	17.00%
Revenue [Member] | Texas [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 30,355	$ 24,737
Percentage of Total Revenue	14.80%	15.80%
Revenue [Member] | Utah [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 20,871	$ 2,534
Percentage of Total Revenue	10.20%	1.60%
Revenue [Member] | California [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 16,666	$ 16,565
Percentage of Total Revenue	8.10%	10.60%
Revenue [Member] | Arizona [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 11,386	$ 7,332
Percentage of Total Revenue	5.60%	4.70%
Revenue [Member] | All Other States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 60,037	$ 51,465
Percentage of Total Revenue	29.30%	32.90%
Revenue [Member] | United States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 166,255	$ 129,217
Percentage of Total Revenue	81.10%	82.60%
Revenue [Member] | Germany [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 37,665	$ 26,190
Percentage of Total Revenue	18.40%	16.70%
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	$ 1,126	$ 990
Percentage of Total Revenue	0.50%	0.70%
Revenue [Member] | International [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 38,791	$ 27,180
Percentage of Total Revenue	18.90%	17.40%