Real Estate and Lending Activities - Schedule of Gross Assets by Geographic Areas (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 6,095,337	$ 5,609,351
Total Gross Assets [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 7,217,777	$ 6,035,922
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	$ 1,250,000
Percentage of Total Gross Assets	17.30%
Total Gross Assets [Member] | Texas [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 944,028	$ 1,060,990
Percentage of Total Gross Assets	13.10%	17.60%
Total Gross Assets [Member] | California [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 542,892	$ 547,085
Percentage of Total Gross Assets	7.50%	9.10%
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	$ 2,669,401	$ 3,047,204
Percentage of Total Gross Assets	37.00%	50.50%
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	$ 251,587	$ 177,317
Percentage of Total Gross Assets	3.50%	2.90%
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	$ 5,657,908	$ 4,832,596
Percentage of Total Gross Assets	78.40%	80.10%
Total Gross Assets [Member] | Germany [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,376,626	$ 1,031,039
Percentage of Total Gross Assets	19.10%	17.10%
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	$ 156,226	$ 161,317
Percentage of Total Gross Assets	2.10%	2.70%
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	$ 27,017	$ 10,970
Percentage of Total Gross Assets	0.40%	0.10%
Total Gross Assets [Member] | International [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,559,869	$ 1,203,326
Percentage of Total Gross Assets	21.60%	19.90%