Reinsurance - Schedule of Catastrophe Losses by Event Type (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) catastrophic_event	Sep. 30, 2017 USD ($) catastrophic_event	Sep. 30, 2018 USD ($) catastrophic_event	Sep. 30, 2017 USD ($) catastrophic_event
Named and numbered storms
Liability for Catastrophe Claims
Number of Catastrophic Events | catastrophic_event	3	4	4	4
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 25,019	$ 83,460	$ 26,233	$ 83,724
Catastrophic Events, Impact on Combined Ratio	14.60%	54.70%	5.20%	20.00%
All other catastrophe loss events
Liability for Catastrophe Claims
Number of Catastrophic Events | catastrophic_event	6	15	22	15
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 9,574	$ (845)	$ 32,017	$ 31,301
Catastrophic Events, Impact on Combined Ratio	5.60%	(0.60%)	6.30%	7.50%
Catastrophe
Liability for Catastrophe Claims
Number of Catastrophic Events | catastrophic_event	9	19	26	19
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 34,593	$ 82,615	$ 58,250	$ 115,025
Catastrophic Events, Impact on Combined Ratio	20.20%	54.10%	11.50%	27.50%