SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Standards (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Percentage Of Fixed Income Portfolio Rated A Or Better	84.50%		84.50%
Operating Lease, Right-of-Use Asset	$ 23,604		$ 23,604
Operating lease liabilities	25,787		25,787
Allocated Share-based Compensation Expense	1,200	$ 1,200	3,600	$ 3,400
Restricted Stock Units (RSUs) [Member]
Allocated Share-based Compensation Expense	300	200	600	400
Outside director | Restricted Stock Units (RSUs) [Member]
Allocated Share-based Compensation Expense	$ 100	$ 100	$ 500	$ 200