RESERVES FOR LOSSES AND LOSS SETTLEMENT EXPENSES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross liability for losses and loss settlement expenses at beginning of year	$ 1,123,896	$ 1,003,895	$ 969,437
Ceded losses and loss settlement expenses	(59,794)	(54,653)	(63,757)
Net liability for losses and loss settlement expenses at beginning of year	1,064,102	949,242	905,680
Losses and loss settlement expenses incurred for claims occurring during
Current year	779,966	683,662	560,482
Prior years	(54,253)	(31,229)	(40,395)
Total incurred	725,713	652,433	520,087
Losses and loss settlement expense payments for claims occurring during
Current year	311,972	277,053	225,022
Prior years	313,531	260,520	251,503
Total paid	625,503	537,573	476,525
Net liability for losses and loss settlement expenses at end of year	1,164,312	1,064,102	949,242
Ceded loss and loss settlement expenses	59,871	59,794	54,653
Gross liability for losses and loss settlement expenses at end of year	$ 1,224,183	$ 1,123,896	$ 1,003,895