Mortgage Loans - Age Analysis of Past Due Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Past Due [Line Items]
Total past due	$ 55,918	$ 22,888
Current	5,060,259	5,123,152
Gross loan receivable amount	5,116,177	5,146,040
Allowance for loan losses	(19,160)	(21,333)	$ (18,866)
Total, net of allowance	$ 5,097,017	$ 5,124,707
Total loans percent	100.00%	100.00%
Hotel
Financing Receivable, Past Due [Line Items]
Total past due	$ 0	$ 22,888
Current	901,044	806,878
Gross loan receivable amount	$ 901,044	$ 829,766
Total loans percent	17.60%	16.10%
Industrial
Financing Receivable, Past Due [Line Items]
Total past due	$ 17,167	$ 0
Current	589,722	761,294
Gross loan receivable amount	$ 606,889	$ 761,294
Total loans percent	11.90%	14.80%
Office
Financing Receivable, Past Due [Line Items]
Total past due	$ 22,870	$ 0
Current	1,587,591	1,747,926
Gross loan receivable amount	$ 1,610,461	$ 1,747,926
Total loans percent	31.50%	34.00%
Retail
Financing Receivable, Past Due [Line Items]
Total past due	$ 4,122	$ 0
Current	843,466	896,429
Gross loan receivable amount	$ 847,588	$ 896,429
Total loans percent	16.50%	17.40%
Other
Financing Receivable, Past Due [Line Items]
Total past due	$ 11,759	$ 0
Current	1,138,436	910,625
Gross loan receivable amount	$ 1,150,195	$ 910,625
Total loans percent	22.50%	17.70%
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due	$ 34,629	$ 0
30-59 Days Past Due | Hotel
Financing Receivable, Past Due [Line Items]
Total past due	0	0
30-59 Days Past Due | Industrial
Financing Receivable, Past Due [Line Items]
Total past due	0	0
30-59 Days Past Due | Office
Financing Receivable, Past Due [Line Items]
Total past due	22,870	0
30-59 Days Past Due | Retail
Financing Receivable, Past Due [Line Items]
Total past due	0	0
30-59 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Total past due	11,759	0
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due	13,076	4,000
60-89 Days Past Due | Hotel
Financing Receivable, Past Due [Line Items]
Total past due	0	4,000
60-89 Days Past Due | Industrial
Financing Receivable, Past Due [Line Items]
Total past due	13,076	0
60-89 Days Past Due | Office
Financing Receivable, Past Due [Line Items]
Total past due	0	0
60-89 Days Past Due | Retail
Financing Receivable, Past Due [Line Items]
Total past due	0	0
60-89 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Total past due	0	0
More Than 90 Days
Financing Receivable, Past Due [Line Items]
Total past due	8,213	18,888
More Than 90 Days | Hotel
Financing Receivable, Past Due [Line Items]
Total past due	0	18,888
More Than 90 Days | Industrial
Financing Receivable, Past Due [Line Items]
Total past due	4,091	0
More Than 90 Days | Office
Financing Receivable, Past Due [Line Items]
Total past due	0	0
More Than 90 Days | Retail
Financing Receivable, Past Due [Line Items]
Total past due	4,122	0
More Than 90 Days | Other
Financing Receivable, Past Due [Line Items]
Total past due	$ 0	$ 0