Debt Service Coverage Ratio for Floating Rate Commercial Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	$ 5,903	$ 6,140
% of total	100.00%	100.00%
Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	262	284
% of total	100.00%	100.00%
Weighted-average loan-to-value	67.00%	72.00%
Retail
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	1,882	1,898
% of total	32.00%	31.00%
Retail | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	105	105
Industrial
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	1,633	1,707
% of total	27.00%	28.00%
Industrial | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	6	8
Office
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	1,533	1,590
% of total	26.00%	26.00%
Office | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	84	84
Apartments
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	578	641
% of total	10.00%	10.00%
Apartments | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment		1
Mixed Use/Other
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	277	304
% of total	5.00%	5.00%
Mixed Use/Other | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	67	86
Less Than 1.00 | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
% of total
Weighted-average loan-to-value
Less Than 1.00 | Retail | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
Less Than 1.00 | Industrial | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
Less Than 1.00 | Office | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
Less Than 1.00 | Apartments | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
Less Than 1.00 | Mixed Use/Other | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.00 - 1.25 | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
% of total
Weighted-average loan-to-value
1.00 - 1.25 | Retail | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.00 - 1.25 | Industrial | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.00 - 1.25 | Office | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.00 - 1.25 | Apartments | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.00 - 1.25 | Mixed Use/Other | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.26 - 1.50 | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	9	9
% of total	3.00%	3.00%
Weighted-average loan-to-value	54.00%	54.00%
1.26 - 1.50 | Retail | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	1	1
1.26 - 1.50 | Industrial | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.26 - 1.50 | Office | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	8	8
1.26 - 1.50 | Apartments | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.26 - 1.50 | Mixed Use/Other | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.51 - 2.00 | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment		5
% of total		2.00%
Weighted-average loan-to-value		44.00%
1.51 - 2.00 | Retail | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.51 - 2.00 | Industrial | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment		5
1.51 - 2.00 | Office | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.51 - 2.00 | Apartments | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
1.51 - 2.00 | Mixed Use/Other | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment
Greater Than 2.00 | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	253	270
% of total	97.00%	95.00%
Weighted-average loan-to-value	68.00%	74.00%
Greater Than 2.00 | Retail | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	104	104
Greater Than 2.00 | Industrial | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	6	3
Greater Than 2.00 | Office | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	76	76
Greater Than 2.00 | Apartments | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment		1
Greater Than 2.00 | Mixed Use/Other | Floating Rate Commercial Mortgage Loans
Commercial Mortgage Loans by Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	$ 67	$ 86