Catastrophe Losses (Details) - Catastrophe $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) catastrophic_event	Dec. 31, 2016 USD ($) catastrophic_event	Dec. 31, 2015 USD ($) catastrophic_event
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	22	19	14
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 116,424	$ 55,842	$ 28,565
Catastrophic Events, Impact on Combined Ratio	19.90%	12.20%	8.50%
Named and numbered storms [Member]
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	6	4	2
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 84,226	$ 33,817	$ 1,167
Catastrophic Events, Impact on Combined Ratio	14.40%	7.40%	0.30%
All other catastrophe loss events [Member]
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	16	15	12
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 32,198	$ 22,025	$ 27,398
Catastrophic Events, Impact on Combined Ratio	5.50%	4.80%	8.20%