Debt Service Coverage Ratio for Fixed Rate Commercial Mortgage Loans by Property Type (Detail) - USD ($) $ in Millions	12 Months Ended
	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%
Fixed Rate Commercial Mortgage Loans
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%
Weighted-average loan-to-value	54.00%	54.00%
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%
Retail | Fixed Rate Commercial Mortgage Loans
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
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%
Industrial | Fixed Rate Commercial Mortgage Loans
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
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%
Office | Fixed Rate Commercial Mortgage Loans
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
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%
Apartments | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 524	$ 495
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%
Mixed Use | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 281	$ 254
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%
Other | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 266	$ 281
Less than 1.00 | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 190	$ 142
% of total	3.00%	2.00%
Weighted-average loan-to-value	59.00%	57.00%
Less than 1.00 | Retail | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 68	$ 43
Less than 1.00 | Industrial | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	24	22
Less than 1.00 | Office | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	44	57
Less than 1.00 | Apartments | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	16	4
Less than 1.00 | Mixed Use | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	4	3
Less than 1.00 | Other | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	34	13
1.00 - 1.25 | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 461	$ 465
% of total	7.00%	7.00%
Weighted-average loan-to-value	62.00%	61.00%
1.00 - 1.25 | Retail | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 140	$ 157
1.00 - 1.25 | Industrial | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	51	75
1.00 - 1.25 | Office | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	89	56
1.00 - 1.25 | Apartments | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	18	24
1.00 - 1.25 | Mixed Use | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	16	19
1.00 - 1.25 | Other | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	147	134
1.26 - 1.50 | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 1,276	$ 1,042
% of total	18.00%	16.00%
Weighted-average loan-to-value	63.00%	62.00%
1.26 - 1.50 | Retail | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 594	$ 448
1.26 - 1.50 | Industrial | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	221	233
1.26 - 1.50 | Office | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	275	156
1.26 - 1.50 | Apartments | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	129	104
1.26 - 1.50 | Mixed Use | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	37	51
1.26 - 1.50 | Other | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	20	50
1.51 - 2.00 | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 2,853	$ 2,950
% of total	41.00%	44.00%
Weighted-average loan-to-value	58.00%	59.00%
1.51 - 2.00 | Retail | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 1,140	$ 1,234
1.51 - 2.00 | Industrial | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	655	653
1.51 - 2.00 | Office | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	748	765
1.51 - 2.00 | Apartments | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	172	168
1.51 - 2.00 | Mixed Use | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	107	80
1.51 - 2.00 | Other | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	31	50
Greater than 2.00 | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 2,153	$ 2,101
% of total	31.00%	31.00%
Weighted-average loan-to-value	41.00%	42.00%
Greater than 2.00 | Retail | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 633	$ 581
Greater than 2.00 | Industrial | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	709	676
Greater than 2.00 | Office | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	471	514
Greater than 2.00 | Apartments | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	189	195
Greater than 2.00 | Mixed Use | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	117	101
Greater than 2.00 | Other | Fixed Rate Commercial Mortgage Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 34	$ 34