Reinsurance - Schedule of Catastrophe Losses by Event Type (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) catastrophic_event	Sep. 30, 2016 USD ($) catastrophic_event	Sep. 30, 2017 USD ($) catastrophic_event	Sep. 30, 2016 USD ($) catastrophic_event
Named and numbered storms
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	4	3	4	3
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 83,460	$ 2,600	$ 83,724	$ 4,069
Catastrophic Events, Impact on Combined Ratio	54.70%	2.20%	20.00%	1.20%
All other catastrophe loss events
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	15	15	15	15
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ (845)	$ 2,509	$ 31,301	$ 19,816
Catastrophic Events, Impact on Combined Ratio	(0.60%)	2.00%	7.50%	5.90%
Catastrophe
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	19	18	19	18
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 82,615	$ 5,109	$ 115,025	$ 23,885
Catastrophic Events, Impact on Combined Ratio	54.10%	4.20%	27.50%	7.10%