Mortgage Loans - Age Analysis of Past Due Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total past due	$ 35,561	$ 55,918
Current	5,149,249	5,060,259
Mortgage loans on real estate	$ 5,184,810	$ 5,116,177
Total loans percent	100.00%	100.00%
Allowance for credit losses	$ (59,445)	$ (19,160)
Total, net of allowance	5,125,365	5,097,017
Apartment
Financing Receivable, Past Due [Line Items]
Total past due	0	0
Current	432,155	416,865
Mortgage loans on real estate	$ 432,155	$ 416,865
Total loans percent	8.30%	8.10%
Allowance for credit losses	$ (2,555)
Hotel
Financing Receivable, Past Due [Line Items]
Total past due	0	$ 0
Current	899,012	901,044
Mortgage loans on real estate	$ 899,012	$ 901,044
Total loans percent	17.30%	17.60%
Allowance for credit losses	$ (26,563)
Industrial
Financing Receivable, Past Due [Line Items]
Total past due	17,159	$ 17,167
Current	638,947	589,721
Mortgage loans on real estate	$ 656,106	$ 606,888
Total loans percent	12.70%	11.90%
Allowance for credit losses	$ (3,849)
Office
Financing Receivable, Past Due [Line Items]
Total past due	13,765	$ 22,870
Current	1,557,472	1,587,591
Mortgage loans on real estate	$ 1,571,237	$ 1,610,461
Total loans percent	30.30%	31.50%
Allowance for credit losses	$ (5,891)
Retail
Financing Receivable, Past Due [Line Items]
Total past due	0	$ 4,122
Current	857,863	843,467
Mortgage loans on real estate	$ 857,863	$ 847,589
Total loans percent	16.60%	16.60%
Allowance for credit losses	$ (5,650)
Other
Financing Receivable, Past Due [Line Items]
Total past due	4,637	$ 11,759
Current	763,800	721,571
Mortgage loans on real estate	$ 768,437	$ 733,330
Total loans percent	14.80%	14.30%
Allowance for credit losses	$ (14,937)
30 - 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due	22,240	$ 34,629
30 - 59 Days Past Due | Apartment
Financing Receivable, Past Due [Line Items]
Total past due	0	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	13,068	0
30 - 59 Days Past Due | Office
Financing Receivable, Past Due [Line Items]
Total past due	4,535	22,870
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	4,637	11,759
60 - 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due	9,230	13,076
60 - 89 Days Past Due | Apartment
Financing Receivable, Past Due [Line Items]
Total past due	0	0
60 - 89 Days Past Due | Hotel
Financing Receivable, Past Due [Line Items]
Total past due	0	0
60 - 89 Days Past Due | Industrial
Financing Receivable, Past Due [Line Items]
Total past due	0	13,076
60 - 89 Days Past Due | Office
Financing Receivable, Past Due [Line Items]
Total past due	9,230	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	4,091	8,213
More Than 90 Days | Apartment
Financing Receivable, Past Due [Line Items]
Total past due	0	0
More Than 90 Days | Hotel
Financing Receivable, Past Due [Line Items]
Total past due	0	0
More Than 90 Days | Industrial
Financing Receivable, Past Due [Line Items]
Total past due	4,091	4,091
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	4,122
More Than 90 Days | Other
Financing Receivable, Past Due [Line Items]
Total past due	$ 0	$ 0