Reinsurance Catastrophe reinsurance Florida (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) hurricane
Reinsurance Retention Policy [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 4,550
Retention	$ 500
Number of hurricanes | hurricane	2
Florida Excess Catastrophe Reinsurance agreement
Reinsurance Retention Policy [Line Items]
Catastrophe reinsurance aggregate, loss limit per contract year, percent	90.00%
Catastrophe reinsurance loss amount	$ 191
Florida Excess Catastrophe Reinsurance Agreement, Provisional
Reinsurance Retention Policy [Line Items]
Limit of aggregate losses under aggregate excess agreement	40
Retention	$ 20
Catastrophe reinsurance aggregate, percent	100.00%
Florida Excess Catastrophe Reinsurance Agreement, Personal Property Lines
Reinsurance Retention Policy [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 254
Retention	20
Florida Excess Catastrophe Reinsurance Agreement, Two Largest Hurricanes
Reinsurance Retention Policy [Line Items]
Limit of aggregate losses under aggregate excess agreement	60
Retention	20
Property Claims Services Class C Excess Catastrophe Reinsurance Agreement Coverage with Portion of Ninth Layer
Reinsurance Retention Policy [Line Items]
Limit of aggregate losses under aggregate excess agreement	115
Retention	3,750
Sanders Re 2014-2 contract
Reinsurance Retention Policy [Line Items]
Limit of aggregate losses under aggregate excess agreement	200
Retention	60
Catastrophe reinsurance loss amount	$ 425
Catastrophe reinsurance aggregate, percent	100.00%