ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Changes in AOCI by Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 12,116,623	$ 10,277,047	$ 11,206,826	$ 7,042,788	$ 7,042,788
Other comprehensive income (loss) on investments for which a credit loss has been recognized in earnings	27,426	0	25,659	0
Ending Balance	12,713,941	11,291,919	12,713,941	11,291,919	11,206,826
Total Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	2,216,469	734,335	1,407,642	(1,404,216)	(1,404,216)
Other comprehensive income (loss) before reclassifications			1,258,066		2,824,107
Other comprehensive income (loss) on investments for which a credit loss has been recognized in earnings					(3,574)
Other comprehensive income (loss) on investments for which a credit loss has been recognized in earnings			25,659
Amounts reclassified from accumulated other comprehensive income (loss)			62,658		(8,675)
Ending Balance	2,754,025	$ 1,558,783	2,754,025	1,558,783	1,407,642
Unrealized Gains and Losses on Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			1,415,631	(1,404,209)	(1,404,209)
Other comprehensive income (loss) before reclassifications			1,258,943		2,833,888
Other comprehensive income (loss) on investments for which a credit loss has been recognized in earnings					(3,574)
Other comprehensive income (loss) on investments for which a credit loss has been recognized in earnings			25,659
Amounts reclassified from accumulated other comprehensive income (loss)			60,231		(10,474)
Ending Balance	2,760,464		2,760,464		1,415,631
Offset of net unrealized losses in AOCI due to impact those net unrealized losses would have on certain of the Company's insurance assets and liabilities had the net unrealized losses been recognized in net income			(1,600,000)		(776,900)
Accumulated Gain and Loss Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			(7,989)	$ (7)	(7)
Other comprehensive income (loss) before reclassifications			(877)		(9,781)
Other comprehensive income (loss) on investments for which a credit loss has been recognized in earnings					0
Other comprehensive income (loss) on investments for which a credit loss has been recognized in earnings			0
Amounts reclassified from accumulated other comprehensive income (loss)			2,427		1,799
Ending Balance	$ (6,439)		$ (6,439)		$ (7,989)