Investments - Net investment income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Investment Income [Line Items]
Investment income, before expense	$ 3,066	$ 3,436	$ 3,514
Investment expense	(213)	(277)	(274)
Net investment income	2,853	3,159	3,240
Fixed income securities
Net Investment Income [Line Items]
Investment income, before expense	2,136	2,175	2,077
Equity securities
Net Investment Income [Line Items]
Investment income, before expense	98	206	170
Mortgage loans
Net Investment Income [Line Items]
Investment income, before expense	220	220	217
Limited partnership interests
Net Investment Income [Line Items]
Investment income, before expense	338	471	705
Short-term investments
Net Investment Income [Line Items]
Investment income, before expense	23	102	73
Other
Net Investment Income [Line Items]
Investment income, before expense	$ 251	$ 262	$ 272