Catastrophe Losses (Details) - Catastrophe $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) catastrophic_event	Dec. 31, 2015 USD ($) catastrophic_event	Dec. 31, 2014 USD ($) catastrophic_event
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	19	14	3
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 55,842	$ 28,565	$ 829
Catastrophic Events, Impact on Combined Ratio	12.20%	8.50%	0.30%
Catastrophe Loss Size, Range 3
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	1	2
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 29,987	$ 11,523
Catastrophic Events, Impact on Combined Ratio	6.50%	3.40%
Catastrophe Loss Size, Range 2
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	12	7
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 21,506	$ 14,699
Catastrophic Events, Impact on Combined Ratio	4.70%	4.40%
Catastrophe Loss Size, Range 1
Liability for Catastrophe Claims [Line Items]
Number of Catastrophic Events | catastrophic_event	6	5	3
Catastrophic Event, Incurred Losses and Loss Adjustment Expense | $	$ 4,349	$ 2,343	$ 829
Catastrophic Events, Impact on Combined Ratio	1.00%	0.70%	0.30%