Real Estate and Lending Activities - Schedule of Revenue from External Customers by Geographic Areas (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue	$ 201,902	$ 166,807	$ 406,948	$ 323,204
Revenue [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 406,948	$ 323,204
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			$ 54,433	$ 53,159
Percentage of Total Revenue			13.40%	16.50%
Revenue [Member] | Texas [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 60,361	$ 49,851
Percentage of Total Revenue			14.80%	15.40%
Revenue [Member] | Utah [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 41,734	$ 5,057
Percentage of Total Revenue			10.30%	1.60%
Revenue [Member] | California [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 29,422	$ 33,123
Percentage of Total Revenue			7.20%	10.30%
Revenue [Member] | Arizona [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 23,286	$ 15,542
Percentage of Total Revenue			5.70%	4.80%
Revenue [Member] | All Other States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 121,307	$ 109,875
Percentage of Total Revenue			29.80%	33.90%
Revenue [Member] | United States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 330,543	$ 266,607
Percentage of Total Revenue			81.20%	82.50%
Revenue [Member] | Germany [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 74,176	$ 54,576
Percentage of Total Revenue			18.20%	16.90%
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			$ 2,229	$ 2,021
Percentage of Total Revenue			0.60%	0.60%
Revenue [Member] | International [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Revenue			$ 76,405	$ 56,597
Percentage of Total Revenue			18.80%	17.50%