Subsequent Event - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Nov. 03, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Subsequent Event [Line Items]
Estimated pretax catastrophe losses, net of reinsurance and inclusive of reinstatement premiums and other associated offsets		$ 29	$ 178	$ 457	$ 470
Estimated pretax catastrophe losses, net of reinsurance and inclusive of reinstatement premiums and other associated offsets (USD per share)		$ 0.13	$ 1.23	$ 3.44	$ 2.98
Fire | Subsequent Event
Subsequent Event [Line Items]
Estimated pretax catastrophe losses, net of reinsurance and inclusive of reinstatement premiums and other associated offsets (USD per share)	$ 0.18
Fire | Minimum | Subsequent Event
Subsequent Event [Line Items]
Estimated pretax catastrophe losses, net of reinsurance and inclusive of reinstatement premiums and other associated offsets	$ 20
Fire | Maximum | Subsequent Event
Subsequent Event [Line Items]
Estimated pretax catastrophe losses, net of reinsurance and inclusive of reinstatement premiums and other associated offsets	$ 25