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, 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	$ 6,943	$ 6,700
% of total	100.00%	100.00%
Weighted-average loan-to-value	64.00%
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	$ 6,943	$ 6,700
% of total	100.00%	100.00%
Weighted-average loan-to-value		54.00%
Retail
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 2,548	$ 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	$ 2,548	$ 2,463
Industrial
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 1,678	$ 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	$ 1,678	$ 1,659
Office
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 1,671	$ 1,548
% of total	24.00%	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	$ 1,671	$ 1,548
Apartments
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 520	$ 495
% of total	7.00%	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	$ 520	$ 495
Mixed Use
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 254	$ 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	$ 254	$ 254
Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Commercial mortgage loans, recorded investment	$ 272	$ 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	$ 272	$ 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	$ 130	$ 142
% of total	2.00%	2.00%
Weighted-average loan-to-value		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	$ 35	$ 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	22	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	53	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	4	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	3	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	13	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	$ 455	$ 465
% of total	7.00%	7.00%
Weighted-average loan-to-value		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	$ 155	$ 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	69	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	56	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	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	18	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	133	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	$ 1,246	$ 1,042
% of total	18.00%	16.00%
Weighted-average loan-to-value		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	$ 572	$ 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	259	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	203	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	108	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	52	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	52	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	$ 3,030	$ 2,950
% of total	43.00%	44.00%
Weighted-average loan-to-value		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	$ 1,216	$ 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	667	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	836	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	191	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	80	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	40	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	$ 2,082	$ 2,101
% of total	30.00%	31.00%
Weighted-average loan-to-value		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	$ 570	$ 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	661	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	523	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	193	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	101	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