Reinsurance and Indemnification - Catastrophe reinsurance (Details) - Catastrophe Reinsurance Program $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) contract reinstatement
Ceded Credit Risk [Line Items]
Catastrophe reinsurance loss amount	$ 2,000
Catastrophe reinsurance aggregate loss limit	4,980
Retention amount for catastrophe reinsurance agreement contracts	500
Limit of aggregate losses under aggregate excess agreement	400
Loss from catastrophes	$ 2,250
Number of reinstatements each year for each contract | reinstatement	1
Contracts number | contract	2
Traditional Market
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 3,000
Insurance Linked Securities (ILS)
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	1,530
Contract One
Ceded Credit Risk [Line Items]
Loss from catastrophes	$ 462
Term of catastrophe reinsurance agreement contract	7 years
Contract Two
Ceded Credit Risk [Line Items]
Loss from catastrophes	$ 284
Contracts number | contract	2
Term of catastrophe reinsurance agreement contract	1 year
Minimum
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	$ 100
Reinsurance ceded amount	150
Maximum
Ceded Credit Risk [Line Items]
Reinsurance ceded amount	$ 375