SIGNIFICANT ACCOUNTING POLICIES AND PRACTICES (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Significant Accounting Policies And Practices [Line Items]
Increase in loss and loss adjustment expenses			$ 303,065	$ 30,146
Decrease in deferred policy acquisistion costs			1,481	25,699
Pre-tax of excess tax deficiencies	$ (4,223)	$ 102	350	6,594
Accounting Standards Update 2016-09 [Member]
Significant Accounting Policies And Practices [Line Items]
Pre-tax of excess tax deficiencies	$ 200		400
Decrease in cash provided by operating activities			$ (200)
Increase in weighted average number of diluted common shares outstanding	9,334		14,102
Reclassifications Representing Corrections of Immaterial Errors [Member]
Significant Accounting Policies And Practices [Line Items]
Decrease in other income		(2,300)		(8,100)
Increase in loss and loss adjustment expenses		2,400		6,000	$ 400
Decrease in commissions and other underwriting expenses		$ (4,600)		$ (14,200)
Decrease in deferred policy acquisistion costs					(1,500)
Decrease in other liabilities					$ (1,900)
Increase in net loss ratio		3.40%		3.30%
Decrease in net expense ratio		(6.70%)		(7.70%)
Decrease in combined ratio		(3.30%)		(4.40%)