Mortgage Loans - Distribution Based on Carrying Amount of Mortgage Loans by Location (Detail) - Mortgage Loans On Real Estate Concentration Risk - Geographic Concentration Risk	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Concentration Risk [Line Items]
Distribution, percentage	100.00%	100.00%
East North Central
Concentration Risk [Line Items]
Distribution, percentage	13.90%	15.40%
East South Central
Concentration Risk [Line Items]
Distribution, percentage	2.80%	3.10%
Mountain
Concentration Risk [Line Items]
Distribution, percentage	20.00%	14.00%
Pacific
Concentration Risk [Line Items]
Distribution, percentage	16.20%	16.50%
South Atlantic
Concentration Risk [Line Items]
Distribution, percentage	12.10%	14.10%
West South Central
Concentration Risk [Line Items]
Distribution, percentage	27.20%	29.80%
Other
Concentration Risk [Line Items]
Distribution, percentage	7.80%	7.10%