BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($)	6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update	us-gaap:AccountingStandardsUpdate201613Member
Stockholders' Equity Attributable to Parent	$ 5,297,820,000			$ 5,544,008,000	$ 5,566,477,000
Premiums receivable	3,527,147,000			3,071,390,000
Allowance for credit loss	10,041,000			7,000,000
Reinsurance recoverable	4,160,521,000			3,877,756,000	3,564,812,000		$ 3,501,669,000
Allowance for credit losses on reinsurance recoverable for unpaid losses and loss expenses	20,987,000			18,000,000
Mortgage loans, held for investment, at fair value	524,757,000			432,748,000
Credit losses associated with commercial mortgage loans	0			0
Fixed maturities, available for sale, allowance for credit loss	$ 6,257,000	$ 20,019,000		$ 0	$ 0	$ 0	$ 0
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent			$ 0