SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Standards (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Mar. 31, 2018	Jan. 01, 2018
Accumulated other comprehensive earnings	$ (14,648)			$ (14,648)		$ 157,919
Retained earnings	953,526			953,526		788,522
Net unrealized losses on equity securities	(12,611)			(39,383)
Expected Value Of Asset And Liability Addition From Adoption Of ASU 2016-02	$ 31,000			$ 31,000
Expense associated with operating leases						$ 7,000
Percentage Of Fixed Income Portfolio Rated A Or Better	82.50%			82.50%
Tax rate used (as a percent)	21.00%	35.00%		21.00%	35.00%
Allocated Share-based Compensation Expense	$ 1,300	$ 1,200		$ 2,300	$ 2,000
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)
Retained earnings								$ 142,200
Net unrealized losses on equity securities				$ (39,400)
Unrealized Gain (Loss) Net Of Tax On Equity Securities Recognized in Other Comprehensive Income					8,500
Reduction to income tax expense				$ 8,300
Accounting Standards Update 2018-02
Accumulated other comprehensive earnings							$ 3,700
Retained earnings							$ (3,700)
Tax rate used (as a percent)				21.00%		35.00%
Restricted Stock Units (RSUs) [Member]
Allocated Share-based Compensation Expense	100		$ 200	$ 200	$ 200
Outside director | Restricted Stock Units (RSUs) [Member]
Allocated Share-based Compensation Expense	$ 100			$ 100