Catastrophe Reinsurance - Coverage for Catastrophe Reinsurance (Details) - Property and Casualty Insurance [Member] - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Retained [Member]
Liability for Catastrophe Claims [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage	0.00%	0.00%	0.00%
First Layer of Coverage [Member]
Liability for Catastrophe Claims [Line Items]
Reinsurance Retention Policy, Excess Retention, Percentage	95.00%	95.00%	95.00%
Second Layer of Coverage [Member]
Liability for Catastrophe Claims [Line Items]
Reinsurance Retention Policy, Excess Retention, Percentage	31.70%	95.00%	95.00%	63.30%
Minimum [Member] | Retained [Member]
Liability for Catastrophe Claims [Line Items]
Reinsurance Retention Policy, Amount Retained	$ 0	$ 0	$ 0
Minimum [Member] | First Layer of Coverage [Member]
Liability for Catastrophe Claims [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	50,000,000	50,000,000	50,000,000
Minimum [Member] | Second Layer of Coverage [Member]
Liability for Catastrophe Claims [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	150,000,000	150,000,000	150,000,000	$ 150,000,000
Maximum [Member] | Retained [Member]
Liability for Catastrophe Claims [Line Items]
Reinsurance Retention Policy, Amount Retained	50,000,000	50,000,000	50,000,000
Maximum [Member] | First Layer of Coverage [Member]
Liability for Catastrophe Claims [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	150,000,000	150,000,000	150,000,000
Maximum [Member] | Second Layer of Coverage [Member]
Liability for Catastrophe Claims [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	$ 250,000,000	$ 350,000,000	$ 350,000,000	$ 350,000,000