Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 12,818	$ 11,349
Decrease in estimates for claims	6,132	14,873
Adjustment expense	19,000	24,000
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Adjustment expense	19,300	23,600
Favorable reserve development net of premium offsets	26,500	49,400
Reinsurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets	$ 7,200	$ 25,800
Reinsurance reserves ogden discount rate			(0.75%)	2.50%