HISTORICAL LOSS AND LAE DEVELOPMENT - Environmental, Asbestos and Mass Tort Exposures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
UNPAID LOSSES AND LAE AT END OF YEAR
Gross	$ 1,271,503	$ 1,139,337	$ 1,103,785	$ 1,121,040
Net	$ 969,512	851,113	805,941	$ 785,934
Minimum period for which product lines related to majority of reserves have been in runoff	20 years
Asbestos and environmental exposures
LOSS AND LAE PAYMENTS (CUMULATIVE)
Gross	$ 132,883	130,358	119,632
Ceded	(67,507)	(66,644)	(62,463)
Net	65,376	63,714	57,169
UNPAID LOSSES AND LAE AT END OF YEAR
Gross	28,042	28,815	41,062
Ceded	(5,715)	(4,987)	(12,559)
Net	$ 22,327	$ 23,828	$ 28,503