SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019	Jun. 30, 2020	Jan. 01, 2020	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Goodwill	$ 825,511,000	$ 825,511,000	$ 825,511,000
Reinsurance allowance, allowance for credit loss	107,472,000	107,472,000	0
Reinsurance recoverable, write-offs		0
Reinsurance recoveries		0
Gross reinsurance receivables	4,700,000,000	4,700,000,000
Cumulative effect adjustment resulting in a decrease in retained earnings	$ (12,713,941,000)	(12,713,941,000)	$ (11,206,826,000)	$ (12,116,623,000)		$ (11,291,919,000)	$ (10,277,047,000)	$ (7,042,788,000)
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member
Reinsurance Receivables
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Concentration risk percentage	11.00%
Reinsurance Receivables | AM Best Rating
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Concentration risk percentage	65.00%
Reinsurance Receivables | AM Best, A+ Rating
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Concentration risk percentage	74.00%
Reinsurance Receivables | AM Best, A Rating
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Concentration risk percentage	16.00%
Reinsurance Receivables | AM Best, A- Rating
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Concentration risk percentage	10.00%
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting in a decrease in retained earnings	$ (1,694,377,000)	$ (1,694,377,000)	$ (1,533,645,000)	$ (1,634,615,000)		$ (1,467,597,000)	$ (1,277,173,000)	(1,031,465,000)
Cumulative Effect Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reinsurance allowance, allowance for credit loss					$ 96,000,000.0
Cumulative effect adjustment resulting in a decrease in retained earnings			138,100,000					50,804,000
Cumulative Effect Adjustment | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment resulting in a decrease in retained earnings			$ 138,100,000		$ 138,100,000			$ 50,804,000