Mortgage Loans - Age Analysis of Past Due Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 22,888	$ 24,579
Current	5,123,152	4,744,286
Gross loan receivable amount	5,146,040	4,768,865
Allowance for loan losses	$ (21,333)	$ (18,866)	$ (12,490)
Total loans percent	100.00%	100.00%
Total, net of allowance	$ 5,124,707	$ 4,749,999
Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	4,985
Current	761,294	781,385
Gross loan receivable amount	$ 761,294	$ 786,370
Total loans percent	14.80%	16.50%
Office
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 0	$ 19,594
Current	1,747,926	1,764,151
Gross loan receivable amount	$ 1,747,926	$ 1,783,745
Total loans percent	34.00%	37.40%
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 0	$ 0
Current	896,429	750,979
Gross loan receivable amount	$ 896,429	$ 750,979
Total loans percent	17.40%	15.70%
Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 22,888	$ 0
Current	1,717,503	1,447,771
Gross loan receivable amount	$ 1,740,391	$ 1,447,771
Total loans percent	33.80%	30.40%
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due		$ 4,985
30-59 Days Past Due | Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 0	4,985
30-59 Days Past Due | Office
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
30-59 Days Past Due | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
30-59 Days Past Due | Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	4,000	10,713
60-89 Days Past Due | Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
60-89 Days Past Due | Office
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	10,713
60-89 Days Past Due | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
60-89 Days Past Due | Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	4,000	0
More Than 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	18,888	8,881
More Than 90 Days | Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
More Than 90 Days | Office
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	8,881
More Than 90 Days | Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
More Than 90 Days | Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 18,888	$ 0