Mortgage Loans - Age Analysis of Past Due Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total past due	$ 0	$ 22,888
Current	4,955,539	5,123,152
Gross loan receivable amount	4,955,539	5,146,040
Allowance for loan losses	$ (18,934)	$ (21,333)
Total loans percent	100.00%	100.00%
Total, net of allowance	$ 4,936,605	$ 5,124,707
Industrial
Financing Receivable, Past Due [Line Items]
Total past due	0	0
Current	512,812	761,294
Gross loan receivable amount	$ 512,812	$ 761,294
Total loans percent	10.30%	14.80%
Office
Financing Receivable, Past Due [Line Items]
Total past due	$ 0	$ 0
Current	1,610,410	1,747,926
Gross loan receivable amount	$ 1,610,410	$ 1,747,926
Total loans percent	32.50%	34.00%
Retail
Financing Receivable, Past Due [Line Items]
Total past due	$ 0	$ 0
Current	842,237	896,429
Gross loan receivable amount	$ 842,237	$ 896,429
Total loans percent	17.00%	17.40%
Other
Financing Receivable, Past Due [Line Items]
Total past due	$ 0	$ 22,888
Current	1,990,080	1,717,503
Gross loan receivable amount	$ 1,990,080	$ 1,740,391
Total loans percent	40.20%	33.80%
30-59 Days Past Due
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	0	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	0	0
60-89 Days Past Due
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	0	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	4,000
More Than 90 Days
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	0	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	0	0
More Than 90 Days | Other
Financing Receivable, Past Due [Line Items]
Total past due	$ 0	$ 18,888