SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Standards (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated other comprehensive earnings	$ (14,572)				$ (14,572)	$ 157,919
Retained earnings	908,079				908,079	788,522
Net unrealized losses on equity securities	(64,200)	$ 4,848	$ (12,611)	$ (26,772)	(98,735)
Unrealized Gain (Loss) Net Of Tax On Equity Securities Recognized in Other Comprehensive Income						24,000	$ 5,600
Income tax expense (benefit)					3,402	(20,439)	42,162
Assets	3,105,065				3,105,065	2,947,244
Liabilities	$ 2,298,223				2,298,223	2,093,646
Expense associated with operating leases					$ 6,900	$ 6,800	$ 6,400
Percentage Of Fixed Income Portfolio Rated A Or Better	84.00%				84.00%
Tax rate used (as a percent)					21.00%	35.00%	35.00%
Allocated Share-based Compensation Expense					$ 4,500	$ 4,400	$ 4,100
Accounting Standards Update 2014-09
Percent of Revenue Out of Scope of ASU 2014-09					99.00%
Accounting Standards Update 2016-01
Accumulated other comprehensive earnings	$ (142,200)				$ (142,200)
Retained earnings	142,200				142,200
Net unrealized losses on equity securities					98,700
Income tax expense (benefit)					(20,700)
Accounting Standards Update 2018-02
Accumulated other comprehensive earnings	3,700				3,700
Retained earnings	(3,700)				(3,700)
Scenario Forecast Adjustment | Minimum | Accounting Standards Update 2016-02
Assets	25,000				25,000
Liabilities	25,000				25,000
Scenario Forecast Adjustment | Maximum | Accounting Standards Update 2016-02
Assets	30,000				30,000
Liabilities	$ 30,000				30,000
Outside director | Restricted Stock Units (RSUs) [Member]
Allocated Share-based Compensation Expense					$ 300