New accounting pronouncements - Summary of periodic payment annuity and life and health Insurance benefits liabilities (Detail) - ASU 2018-12 [Member] $ in Millions	Jan. 01, 2021 USD ($)
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Reclassifications to Other Policyholder Liabilities	$ (1,215)
Change in discount rate assumptions	8,000
Change in cash flow assumptions	435
Balance as of January 1, 2021	28,836
Previously Reported [Member]
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Balance at December 31, 2020, as previously reported	21,616
Periodic Payment Annuities [Member]
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Reclassifications to Other Policyholder Liabilities	(286)
Change in discount rate assumptions	6,553
Change in cash flow assumptions	(117)
Balance as of January 1, 2021	17,124
Periodic Payment Annuities [Member] | Previously Reported [Member]
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Balance at December 31, 2020, as previously reported	10,974
Life and Health [Member]
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Reclassifications to Other Policyholder Liabilities	(929)
Change in discount rate assumptions	1,447
Change in cash flow assumptions	552
Balance as of January 1, 2021	11,712
Life and Health [Member] | Previously Reported [Member]
Liabilities for Guarantees on Long-Duration Contracts [Line Items]
Balance at December 31, 2020, as previously reported	$ 10,642