Mortgage Loans - Distribution Based on Carrying Amount of Mortgage Loans by Location (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Concentration Risk [Line Items]
Mortgage loans, carrying amount	$ 5,125,365	$ 5,097,017
East North Central
Concentration Risk [Line Items]
Mortgage loans, carrying amount	670,432	667,150
East South Central
Concentration Risk [Line Items]
Mortgage loans, carrying amount	149,651	144,887
Mountain
Concentration Risk [Line Items]
Mortgage loans, carrying amount	1,222,815	1,200,434
Pacific
Concentration Risk [Line Items]
Mortgage loans, carrying amount	889,300	852,574
South Atlantic
Concentration Risk [Line Items]
Mortgage loans, carrying amount	620,658	621,875
West South Central
Concentration Risk [Line Items]
Mortgage loans, carrying amount	1,248,628	1,272,522
Other
Concentration Risk [Line Items]
Mortgage loans, carrying amount	$ 323,881	$ 337,575
Mortgage Loans On Real Estate Concentration Risk | Geographic Concentration Risk
Concentration Risk [Line Items]
Distribution, percentage	100.00%	100.00%
Mortgage Loans On Real Estate Concentration Risk | Geographic Concentration Risk | East North Central
Concentration Risk [Line Items]
Distribution, percentage	13.10%	13.10%
Mortgage Loans On Real Estate Concentration Risk | Geographic Concentration Risk | East South Central
Concentration Risk [Line Items]
Distribution, percentage	2.90%	2.80%
Mortgage Loans On Real Estate Concentration Risk | Geographic Concentration Risk | Mountain
Concentration Risk [Line Items]
Distribution, percentage	23.90%	23.60%
Mortgage Loans On Real Estate Concentration Risk | Geographic Concentration Risk | Pacific
Concentration Risk [Line Items]
Distribution, percentage	17.40%	16.70%
Mortgage Loans On Real Estate Concentration Risk | Geographic Concentration Risk | South Atlantic
Concentration Risk [Line Items]
Distribution, percentage	12.10%	12.20%
Mortgage Loans On Real Estate Concentration Risk | Geographic Concentration Risk | West South Central
Concentration Risk [Line Items]
Distribution, percentage	24.40%	25.00%
Mortgage Loans On Real Estate Concentration Risk | Geographic Concentration Risk | Other
Concentration Risk [Line Items]
Distribution, percentage	6.20%	6.60%