SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Standards (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Percentage Of Fixed Income Portfolio Rated A Or Better	83.00%		83.00%
Operating Lease, Right-of-Use Asset	$ 24,449		$ 24,449
Operating lease liabilities	26,640		26,640
Allocated Share-based Compensation Expense	1,400	$ 1,300	2,400	$ 2,300
Restricted Stock Units (RSUs) [Member]
Allocated Share-based Compensation Expense	200	100	300	200
Outside director | Restricted Stock Units (RSUs) [Member]
Allocated Share-based Compensation Expense	$ 200	$ 100	$ 300	$ 100