Reinsurance and Indemnification - Florida Hurricane Catastrophe Fund (Details)	12 Months Ended
	Dec. 31, 2020 USD ($)	May 31, 2020 USD ($) hurricane	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Reinsurance Retention Policy [Line Items]
Property-liability insurance premiums earned	$ 1,383,000,000		$ 1,407,000,000	$ 1,306,000,000
Florida Hurricane Catastrophe Fund
Reinsurance Retention Policy [Line Items]
Emergency assessment maximum percentage of premiums first year	6.00%
Emergency assessment maximum percentage of premiums subsequent years	10.00%
Emergency assessment, bonds issued				2,000,000,000.00
Emergency assessment maximum percentage of premiums subsequent years, FL OIR	0.00%
Florida Hurricane Catastrophe Fund
Reinsurance Retention Policy [Line Items]
Property-liability insurance premiums earned	$ 9,000,000		9,000,000	10,000,000
Reinsurance recoverable, qualifying losses, amount	15,000,000		33,000,000	$ 143,000,000
Reinsurance ceded amount	118,000,000
Percent of losses in excess of retention level to be reimbursed		90.00%
Retention level for losses related to 2 largest hurricanes		$ 58,000,000
Number of storms | hurricane		2
Retention level for losses related to other hurricanes		$ 19,000,000
Maximum Retention level for losses related to hurricanes		$ 146,000,000
Reinsurance and indemnification recoverables, net	$ 32,000,000		$ 52,000,000