Real Estate and Lending Activities - Schedule of Gross Assets by Geographic Areas (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 8,927,040	$ 6,418,536
Total Gross Assets [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 9,355,935	$ 7,143,121
Percentage of Total Gross Assets	100.00%	100.00%
Total Gross Assets [Member] | Massachusetts [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,284,156	$ 1,250,000
Percentage of Total Gross Assets	13.70%	17.50%
Total Gross Assets [Member] | Texas [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,275,784	$ 947,443
Percentage of Total Gross Assets	13.60%	13.30%
Total Gross Assets [Member] | Utah [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,035,793	$ 107,151
Percentage of Total Gross Assets	11.10%	1.50%
Total Gross Assets [Member] | California [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 542,879	$ 542,889
Percentage of Total Gross Assets	5.80%	7.60%
Total Gross Assets [Member] | Arizona [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 498,844	$ 331,834
Percentage of Total Gross Assets	5.30%	4.60%
Total Gross Assets [Member] | All Other States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 2,506,538	$ 2,234,332
Percentage of Total Gross Assets	26.80%	31.30%
Total Gross Assets [Member] | Other Domestic Assets [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 397,850	$ 264,215
Percentage of Total Gross Assets	4.30%	3.70%
Total Gross Assets [Member] | United States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 7,541,844	$ 5,677,864
Percentage of Total Gross Assets	80.60%	79.50%
Total Gross Assets [Member] | Germany [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,556,392	$ 1,281,649
Percentage of Total Gross Assets	16.60%	17.90%
Total Gross Assets [Member] | United Kingdom, Italy, and Spain [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 203,044	$ 146,920
Percentage of Total Gross Assets	2.20%	2.10%
Total Gross Assets [Member] | Other International Assets [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 54,655	$ 36,688
Percentage of Total Gross Assets	0.60%	0.50%
Total Gross Assets [Member] | International [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,814,091	$ 1,465,257
Percentage of Total Gross Assets	19.40%	20.50%