Losses and Loss Expenses - Asbestos and Environmental Exposures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of opening and closing unpaid losses and loss expenses related to asbestos and environmental exposure claims:
Net unpaid losses and loss expenses at beginning of year	$ 92,834	$ 93,704	$ 81,416
Net incurred losses and loss expenses	(175)	9,042	15,663
Less net paid losses and loss expenses	5,178	9,912	9,087
Net increase (decrease) in unpaid losses and loss expenses	(5,353)	(870)	6,576
Acquired reserves	0	0	5,712
Net unpaid losses and loss expenses at end of year	87,481	92,834	93,704
Unpaid losses and loss expenses recoverable at end of year	83,305	83,430	93,688
Gross unpaid losses and loss expenses at end of year	170,786	176,264	187,392
Reserves for incurred but not reported losses, net of reinsurance	$ 51,300	$ 60,300	$ 65,100