SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Standards (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Percentage Of Fixed Income Portfolio Rated A Or Better	85.00%
Operating Lease, Right-of-Use Asset	$ 22,335,000
Operating Lease Liabilities	24,475,000
Allocated Share-based Compensation Expense	4,500,000	$ 4,500,000	$ 4,400,000
Accounting Standards Update 2016-02
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets				$ 0
Outside director | Restricted Stock Units (RSUs)
Allocated Share-based Compensation Expense	$ 600,000