Real Estate and Loans Receivable - Schedule of Revenue from External Customers by Geographic Areas (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 204,961	$ 176,580	$ 166,807	$ 156,397	$ 153,283	$ 126,555	$ 126,300	$ 134,999	$ 704,745	$ 541,137	$ 441,878
Revenue [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue									$ 704,745	$ 541,137
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									$ 107,195	$ 26,098
Percentage of Total Revenue									15.20%	4.80%
Revenue [Member] | Texas [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue									$ 102,926	$ 96,992
Percentage of Total Revenue									14.60%	17.90%
Revenue [Member] | California [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue									$ 66,241	$ 66,197
Percentage of Total Revenue									9.40%	12.20%
Revenue [Member] | Utah [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue									$ 28,831	$ 9,942
Percentage of Total Revenue									4.10%	1.80%
Revenue [Member] | Arizona [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue									$ 36,393	$ 23,798
Percentage of Total Revenue									5.20%	4.40%
Revenue [Member] | Other states [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue									$ 235,545	$ 216,505
Percentage of Total Revenue									33.40%	40.10%
Revenue [Member] | United States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue									$ 577,131	$ 439,532
Percentage of Total Revenue									81.90%	81.20%
Revenue [Member] | Germany [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue									$ 123,453	$ 97,382
Percentage of Total Revenue									17.50%	18.00%
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									$ 4,161	$ 4,223
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									$ 127,614	$ 101,605
Percentage of Total Revenue									18.10%	18.80%