Reinsurance - Schedule of Catastrophe Losses by Event Type (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) catastrophic_event	Mar. 31, 2017 USD ($) catastrophic_event
Liability for Catastrophe Claims [Line Items]
Current Fiscal Year End Date	--12-31
Named and numbered storms
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	0	0
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 0	$ 0
Catastrophic Events, Impact on Combined Ratio	0.00%	0.00%
All other catastrophe loss events
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	8	5
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 6,317	$ 10,612
Catastrophic Events, Impact on Combined Ratio	3.90%	9.90%
Catastrophe
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	8	5
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 6,317	$ 10,612
Catastrophic Events, Impact on Combined Ratio	3.90%	9.90%