Reinsurance - Schedule of Catastrophe Losses by Event Type (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) catastrophic_event	Jun. 30, 2017 USD ($) catastrophic_event	Jun. 30, 2018 USD ($) catastrophic_event	Jun. 30, 2017 USD ($) catastrophic_event
Named and numbered storms
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	1	1
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 1,214	$ 264
Catastrophic Events, Impact on Combined Ratio	0.70%	0.20%
All other catastrophe loss events
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	8	14
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 16,126	$ 21,534
Catastrophic Events, Impact on Combined Ratio	9.40%	13.50%
Catastrophe
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	9	15	17	15
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 17,340	$ 21,798	$ 23,657	$ 32,410
Catastrophic Events, Impact on Combined Ratio	10.10%	13.70%	7.00%	12.10%
Catastrophe | Named and numbered storms
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event			1	1
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $			$ 1,214	$ 264
Catastrophic Events, Impact on Combined Ratio			0.30%	0.10%
Catastrophe | All other catastrophe loss events
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event			16	14
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $			$ 22,443	$ 32,146
Catastrophic Events, Impact on Combined Ratio			6.70%	12.00%