Losses and Loss Expenses - Asbestos and Environmental Exposures Claims Rollforward (Details) - claims	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Asbestos claims
Reconciliation of opening and closing unpaid losses and loss expenses related to asbestos and environmental exposure claims:
Total number of claims outstanding, beginning of period	2,591	1,399	1,097
New claims reported	579	272	456
Claims resolved	(753)	(246)	(154)
Reserves acquired		1,166
Total number of claims outstanding, end of period	2,417	2,591	1,399
Environmental claims
Reconciliation of opening and closing unpaid losses and loss expenses related to asbestos and environmental exposure claims:
Total number of claims outstanding, beginning of period	430	459	338
New claims reported	69	67	190
Claims resolved	(41)	(96)	(69)
Reserves acquired		0
Total number of claims outstanding, end of period	458	430	459