Reinsurance and Indemnification - California E&S Earthquake Contract (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Ceded Credit Risk [Line Items]
Ceded premium earned	$ 1,407	$ 1,306	$ 1,274
Limit of aggregate losses under aggregate excess agreement	$ 3,400
E&S Earthquake Agreement
Ceded Credit Risk [Line Items]
Term of catastrophe reinsurance agreement contract	3 years
Catastrophe insurance, quota share basis, percentage	100.00%
Limit of aggregate losses under aggregate excess agreement	$ 400