Mortgage Loans - Distribution Based on Carrying Amount of Mortgage Loans by Location (Detail) - Mortgage Loans On Real Estate Concentration Risk - Geographic Concentration Risk	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Concentration Risk [Line Items]
Distribution, percentage	100.00%	100.00%
East North Central
Concentration Risk [Line Items]
Distribution, percentage	13.10%	13.90%
East South Central
Concentration Risk [Line Items]
Distribution, percentage	2.80%	2.80%
Mountain
Concentration Risk [Line Items]
Distribution, percentage	21.10%	20.00%
Pacific
Concentration Risk [Line Items]
Distribution, percentage	17.10%	16.20%
South Atlantic
Concentration Risk [Line Items]
Distribution, percentage	11.90%	12.10%
West South Central
Concentration Risk [Line Items]
Distribution, percentage	26.60%	27.20%
Other
Concentration Risk [Line Items]
Distribution, percentage	7.40%	7.80%