Aging of Past Due Commercial Mortgage Loans by Property Type (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 6,933	$ 6,700
% of total	100.00%	100.00%
Retail
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 2,575	$ 2,463
% of total	37.00%	37.00%
Industrial
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 1,660	$ 1,659
% of total	24.00%	25.00%
Office
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 1,627	$ 1,548
% of total	23.00%	23.00%
Apartments
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 524	$ 495
% of total	8.00%	7.00%
Mixed Use
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 281	$ 254
% of total	4.00%	4.00%
Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 266	$ 281
% of total	4.00%	4.00%
31-60 days past due
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 0	$ 3
% of total	0.00%	0.00%
31-60 days past due | Retail
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 0	$ 3
31-60 days past due | Industrial
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
31-60 days past due | Office
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
31-60 days past due | Apartments
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
31-60 days past due | Mixed Use
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
31-60 days past due | Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
61-90 days past due
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 0	$ 0
% of total	0.00%	0.00%
61-90 days past due | Retail
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 0	$ 0
61-90 days past due | Industrial
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
61-90 days past due | Office
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
61-90 days past due | Apartments
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
61-90 days past due | Mixed Use
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
61-90 days past due | Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
Greater than 90 days past due
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 0	$ 3
% of total	0.00%	0.00%
Greater than 90 days past due | Retail
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 0	$ 0
Greater than 90 days past due | Industrial
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
Greater than 90 days past due | Office
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	3
Greater than 90 days past due | Apartments
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
Greater than 90 days past due | Mixed Use
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
Greater than 90 days past due | Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
Total past due
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 0	$ 6
% of total	0.00%	0.00%
Total past due | Retail
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 0	$ 3
Total past due | Industrial
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
Total past due | Office
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	3
Total past due | Apartments
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
Total past due | Mixed Use
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
Total past due | Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	0	0
Current
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 6,933	$ 6,694
% of total	100.00%	100.00%
Current | Retail
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 2,575	$ 2,460
Current | Industrial
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	1,660	1,659
Current | Office
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	1,627	1,545
Current | Apartments
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	524	495
Current | Mixed Use
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	281	254
Current | Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 266	$ 281