Commercial Mortgage Loans By Year of Origination and Credit Quality Indicator (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	$ 6,787	$ 6,774
Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	6,787	6,774
0% - 50%
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	2,613	2,645
0% - 50% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	2,613
51% - 60%
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	1,598	1,628
51% - 60% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	1,598
61% - 75%
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	2,576	2,396
61% - 75% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	2,576
76% - 100%
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0	103
76% - 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
Greater than 100%
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0	2
Greater than 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
Less than 1.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	207	216
1.00 - 1.25 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	511	526
1.26 - 1.50 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	1,009	992
1.51 - 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	2,645	2,590
Greater than 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	2,415	$ 2,450
2021 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	142
2021 | 0% - 50% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2021 | 51% - 60% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	6
2021 | 61% - 75% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	136
2021 | 76% - 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2021 | Greater than 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2021 | Less than 1.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2021 | 1.00 - 1.25 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2021 | 1.26 - 1.50 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	27
2021 | 1.51 - 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	106
2021 | Greater than 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	9
2020 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	539
2020 | 0% - 50% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	59
2020 | 51% - 60% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	51
2020 | 61% - 75% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	429
2020 | 76% - 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2020 | Greater than 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2020 | Less than 1.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2020 | 1.00 - 1.25 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	53
2020 | 1.26 - 1.50 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	82
2020 | 1.51 - 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	282
2020 | Greater than 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	122
2019 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	779
2019 | 0% - 50% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	28
2019 | 51% - 60% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	84
2019 | 61% - 75% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	667
2019 | 76% - 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2019 | Greater than 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2019 | Less than 1.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	8
2019 | 1.00 - 1.25 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	67
2019 | 1.26 - 1.50 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	242
2019 | 1.51 - 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	293
2019 | Greater than 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	169
2018 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	960
2018 | 0% - 50% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	94
2018 | 51% - 60% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	334
2018 | 61% - 75% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	532
2018 | 76% - 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2018 | Greater than 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2018 | Less than 1.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	27
2018 | 1.00 - 1.25 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	54
2018 | 1.26 - 1.50 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	201
2018 | 1.51 - 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	444
2018 | Greater than 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	234
2017 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	718
2017 | 0% - 50% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	153
2017 | 51% - 60% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	287
2017 | 61% - 75% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	278
2017 | 76% - 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2017 | Greater than 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2017 | Less than 1.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	10
2017 | 1.00 - 1.25 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	42
2017 | 1.26 - 1.50 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	60
2017 | 1.51 - 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	334
2017 | Greater than 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	272
2016 and prior | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	3,649
2016 and prior | 0% - 50% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	2,279
2016 and prior | 51% - 60% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	836
2016 and prior | 61% - 75% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	534
2016 and prior | 76% - 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2016 and prior | Greater than 100% | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	0
2016 and prior | Less than 1.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	162
2016 and prior | 1.00 - 1.25 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	295
2016 and prior | 1.26 - 1.50 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	397
2016 and prior | 1.51 - 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	1,186
2016 and prior | Greater than 2.00 | Fixed Rate Commercial Mortgage Loans [Member]
Disclosure Of Commercial Mortgage Loans By Year Of Origination And Credit Quality Indicator [Line Items]
Commercial mortgage loans, recorded investment	$ 1,609