Reinsurance and Indemnification - National Flood Insurance Program (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reinsurance Retention Policy [Line Items]
Ceded premium earned	$ 1,407,000,000	$ 1,306,000,000	$ 1,274,000,000
Florida Hurricane Catastrophe Fund
Reinsurance Retention Policy [Line Items]
Reinsurance and indemnification recoverables, net	52,000,000	104,000,000
Ceded premium earned	9,000,000	10,000,000	11,000,000
Ceded losses incurred	33,000,000	143,000,000	19,000,000
National Flood Insurance Program
Reinsurance Retention Policy [Line Items]
Reinsurance and indemnification recoverables, net	25,000,000	31,000,000
Ceded premium earned	258,000,000	258,000,000	263,000,000
Ceded losses incurred	$ 150,000,000	$ 118,000,000	$ 1,120,000,000