Real Estate and Lending Activities - Schedule of Gross Assets by Geographic Areas (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 7,327,837	$ 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	$ 9,129,778	$ 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	$ 1,262,041	$ 1,250,000
Percentage of Total Gross Assets	13.80%	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	$ 1,230,945	$ 947,443
Percentage of Total Gross Assets	13.50%	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	$ 1,083,152	$ 107,151
Percentage of Total Gross Assets	11.90%	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	$ 542,883	$ 542,889
Percentage of Total Gross Assets	5.90%	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	$ 486,547	$ 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	$ 2,502,791	$ 2,234,332
Percentage of Total Gross Assets	27.40%	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	$ 352,748	$ 264,215
Percentage of Total Gross Assets	3.90%	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	$ 7,461,107	$ 5,677,864
Percentage of Total Gross Assets	81.70%	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	$ 1,421,350	$ 1,281,649
Percentage of Total Gross Assets	15.60%	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	$ 198,400	$ 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	$ 48,921	$ 36,688
Percentage of Total Gross Assets	0.50%	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	$ 1,668,671	$ 1,465,257
Percentage of Total Gross Assets	18.30%	20.50%