Real Estate and Lending Activities - Schedule of Gross Assets by Geographic Areas (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 9,031,840	$ 9,020,288
Total Gross Assets [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 9,544,607	$ 9,476,212
Percentage of Total Gross Assets	100.00%	100.00%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | Massachusetts [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,298,226	$ 1,297,226
Percentage of Total Gross Assets	13.60%	13.70%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | Texas [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,260,795	$ 1,257,390
Percentage of Total Gross Assets	13.20%	13.30%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | Utah [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,035,748	$ 1,035,501
Percentage of Total Gross Assets	10.90%	10.90%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | California [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 542,873	$ 542,876
Percentage of Total Gross Assets	5.70%	5.70%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | Arizona [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 489,185	$ 491,284
Percentage of Total Gross Assets	5.10%	5.20%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | All Other States [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 2,616,686	$ 2,618,536
Percentage of Total Gross Assets	27.40%	27.60%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | Other Domestic Assets [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 402,841	$ 387,050
Percentage of Total Gross Assets	4.20%	4.10%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | United States [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 7,646,354	$ 7,629,863
Percentage of Total Gross Assets	80.10%	80.50%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | Germany [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,623,755	$ 1,581,726
Percentage of Total Gross Assets	17.00%	16.70%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | United Kingdom, Italy, and Spain [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 211,244	$ 207,014
Percentage of Total Gross Assets	2.20%	2.20%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | Other International States
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 63,254	$ 57,609
Percentage of Total Gross Assets	0.70%	0.60%
Total Gross Assets [Member] | Credit Concentration Risk [Member] | International [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,898,253	$ 1,846,349
Percentage of Total Gross Assets	19.90%	19.50%