Mortgage Loans - Distribution Based on Carrying Amount of Mortgage Loans by Location (Detail) - Mortgage Loans On Real Estate Concentration Risk - Geographic Concentration Risk	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Distribution, percentage	100.00%	100.00%
East North Central
Concentration Risk [Line Items]
Distribution, percentage	13.50%	13.90%
East South Central
Concentration Risk [Line Items]
Distribution, percentage	2.60%	2.80%
Mountain
Concentration Risk [Line Items]
Distribution, percentage	22.30%	20.00%
Pacific
Concentration Risk [Line Items]
Distribution, percentage	17.50%	16.20%
South Atlantic
Concentration Risk [Line Items]
Distribution, percentage	12.30%	12.10%
West South Central
Concentration Risk [Line Items]
Distribution, percentage	25.00%	27.20%
Other
Concentration Risk [Line Items]
Distribution, percentage	6.80%	7.80%