Debt Service Coverage Ratio for Fixed Rate Commercial Mortgage Loans by Property Type (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 6,787	$ 6,774
% 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	$ 6,787	$ 6,774
% of total	100.00%	100.00%
Weighted-average debt-to-value	53.00%	53.00%
Retail
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 2,541	$ 2,442
% of total	37.00%	36.00%
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	$ 2,541	$ 2,442
Industrial
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 1,593	$ 1,638
% of total	24.00%	24.00%
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	$ 1,593	$ 1,638
Office
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 1,539	$ 1,567
% 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	$ 1,539	$ 1,567
Apartments
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 569	$ 529
% of total	8.00%	8.00%
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	$ 569	$ 529
Mixed Use
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 284	$ 286
% 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	$ 284	$ 286
Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 261	$ 312
% of total	4.00%	5.00%
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	$ 261	$ 312
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	$ 207	$ 216
% of total	3.00%	3.00%
Weighted-average debt-to-value	57.00%	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	$ 52	$ 55
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	21	21
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	96	101
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	8	9
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	5	5
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	25	25
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	$ 511	$ 526
% of total	7.00%	8.00%
Weighted-average debt-to-value	61.00%	62.00%
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	$ 162	$ 169
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	80	85
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	97	99
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	24	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	24	24
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	124	125
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	$ 1,009	$ 992
% of total	15.00%	15.00%
Weighted-average debt-to-value	62.00%	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	$ 500	$ 483
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	147	143
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	171	170
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	125	126
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	29	29
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	37	41
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	$ 2,645	$ 2,590
% of total	39.00%	38.00%
Weighted-average debt-to-value	57.00%	57.00%
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	$ 1,063	$ 969
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	582	616
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	621	634
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	226	228
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	129	115
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	24	28
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	$ 2,415	$ 2,450
% of total	36.00%	36.00%
Weighted-average debt-to-value	44.00%	44.00%
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	$ 764	$ 766
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	763	773
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	554	563
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	186	142
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	97	113
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	$ 51	$ 93