Reinsurance - Florida Excess Catastrophe Reinsurance Agreement (Details) $ in Millions	6 Months Ended	7 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) hurricane	Dec. 31, 2017 USD ($) contract limit
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts			$ 500
Number of hurricanes | hurricane		2
Florida Excess Catastrophe Reinsurance Agreement
Ceded Credit Risk [Line Items]
Contracts number | contract			5
Number of contracts, traditional market | contract			2
Catastrophe reinsurance aggregate loss limit, remaining		$ 168	$ 168
Catastrophe reinsurance aggregate, loss limit per contract year, percent	90.00%
Catastrophe reinsurance loss amount	$ 187
Florida Excess Catastrophe Reinsurance Agreement, Provisional
Ceded Credit Risk [Line Items]
Retention percentage of reinsurance limit			100.00%
Number of limits available per contract year | limit			3
Limit of aggregate losses under aggregate excess agreement			$ 38
Retention amount for catastrophe reinsurance agreement contracts			$ 20
Catastrophe reinsurance agreement contracts, number of limits remaining per a contract year | limit			2
Florida Excess Catastrophe Reinsurance Agreement, Two Largest Hurricanes
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement		56
Retention amount for catastrophe reinsurance agreement contracts		19
Florida Excess Catastrophe Reinsurance Agreement, Personal Property Lines
Ceded Credit Risk [Line Items]
Retention percentage of reinsurance limit			100.00%
Limit of aggregate losses under aggregate excess agreement			$ 231
Retention amount for catastrophe reinsurance agreement contracts			20
Catastrophe reinsurance aggregate loss limit, exhausted		27	27
Hurricane | Florida Excess Catastrophe Reinsurance Agreement
Ceded Credit Risk [Line Items]
Catastrophe reinsurance aggregate loss limit, exhausted		19	19
Hurricane | Florida Excess Catastrophe Reinsurance Agreement, Provisional
Ceded Credit Risk [Line Items]
Catastrophe reinsurance aggregate loss limit, exhausted		$ 38	$ 38