Losses and Loss Expenses - Asbestos and Environmental Exposures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	$ 93,704	$ 81,416	$ 80,435
Net incurred losses and loss expenses	9,042	15,663	8,903
Less net paid losses and loss expenses	9,912	9,087	7,922
Net increase (decrease) in unpaid losses and loss expenses	(870)	6,576	981
Acquired reserves	0	5,712	0
Net unpaid losses and loss expenses at end of year	92,834	93,704	81,416
Unpaid losses and loss expenses recoverable at end of year	83,430	93,688	100,537
Gross unpaid losses and loss expenses at end of year	176,264	187,392	181,953
Reserves for incurred but not reported losses, net of reinsurance	$ 60,300	$ 65,100	$ 49,300