Mortgage Loans - Distribution Based on Carrying Amount of Mortgage Loans by Location (Detail) - Mortgage Loans On Real Estate Concentration Risk [Member] - Geographic Concentration Risk [Member]	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Concentration Risk [Line Items]
Mortgage loans	100.00%	100.00%
East North Central [Member]
Concentration Risk [Line Items]
Mortgage loans	18.80%	19.40%
East South Central [Member]
Concentration Risk [Line Items]
Mortgage loans	4.40%	5.00%
Mountain [Member]
Concentration Risk [Line Items]
Mortgage loans	12.30%	11.00%
Pacific [Member]
Concentration Risk [Line Items]
Mortgage loans	10.00%	10.80%
South Atlantic [Member]
Concentration Risk [Line Items]
Mortgage loans	19.70%	21.90%
West South Central [Member]
Concentration Risk [Line Items]
Mortgage loans	29.20%	24.90%
Other [Member]
Concentration Risk [Line Items]
Mortgage loans	5.60%	7.00%