Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 11,349		$ 9,402
Decrease in estimates for claims	14,873		$ 21,696
Adjustment expense	23,600	$ 28,700
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Adjustment expense	49,400	24,800
Reinsurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Adjustment expense	$ 25,800	$ 3,900