Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 6,448	$ 5,761
Decrease in estimates for claims	6,662	4,841
Adjustment expense	12,000	2,000
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Adjustment expense	12,200	2,400
Favorable reserve development net of premium offsets	16,300	26,200
Reinsurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets	$ 4,100	$ 23,800
Reinsurance reserves ogden discount rate			(0.75%)	2.50%