Real Estate and Lending Activities - Schedule of Gross Assets by Geographic Areas (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 6,780,730	$ 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	$ 7,445,822	$ 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	$ 1,250,000	$ 1,250,000
Percentage of Total Gross Assets	16.80%	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	$ 893,749	$ 947,443
Percentage of Total Gross Assets	12.00%	13.30%
Total Gross Assets [Member] | California [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 542,886	$ 542,889
Percentage of Total Gross Assets	7.30%	7.60%
Total Gross Assets [Member] | New Jersey [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 416,490	$ 447,436
Percentage of Total Gross Assets	5.60%	6.30%
Total Gross Assets [Member] | Arizona [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 331,833	$ 331,834
Percentage of Total Gross Assets	4.50%	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	$ 2,175,466	$ 1,894,047
Percentage of Total Gross Assets	29.20%	26.50%
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	$ 284,070	$ 264,215
Percentage of Total Gross Assets	3.80%	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	$ 5,894,494	$ 5,677,864
Percentage of Total Gross Assets	79.20%	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	$ 1,320,487	$ 1,281,649
Percentage of Total Gross Assets	17.70%	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	$ 190,276	$ 146,920
Percentage of Total Gross Assets	2.50%	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	$ 40,565	$ 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	$ 1,551,328	$ 1,465,257
Percentage of Total Gross Assets	20.80%	20.50%