INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Amortized Cost	$ 62,741,533		$ 61,970,744
Gross Unrealized Gains	5,019,654		3,100,992
Gross Unrealized Losses	(518,856)		(325,660)
Allowance for Expected Credit Losses	(82,081)
Fair Value	67,160,250		64,746,076
Residential mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost	6,783,626		5,812,170
Gross Unrealized Gains	231,006		125,493
Gross Unrealized Losses	(503)		(6,322)
Allowance for Expected Credit Losses	0
Fair Value	7,014,129		5,931,341
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost	2,482,700		2,588,575
Gross Unrealized Gains	87,882		54,385
Gross Unrealized Losses	(29,100)		(3,292)
Allowance for Expected Credit Losses	0
Fair Value	2,541,482		2,639,668
Other asset-backed securities
Schedule of Investments [Line Items]
Amortized Cost	1,704,674		1,764,120
Gross Unrealized Gains	26,109		32,041
Gross Unrealized Losses	(28,104)		(14,926)
Allowance for Expected Credit Losses	(655)	$ (658)	0
Fair Value	1,702,024		1,781,235
U.S. government-related securities
Schedule of Investments [Line Items]
Amortized Cost	1,145,220		1,032,048
Gross Unrealized Gains	29,542		5,664
Gross Unrealized Losses	(844)		(5,316)
Allowance for Expected Credit Losses	0
Fair Value	1,173,918		1,032,396
Other government-related securities
Schedule of Investments [Line Items]
Amortized Cost	566,961		548,136
Gross Unrealized Gains	63,570		51,024
Gross Unrealized Losses	(4,361)		(1,991)
Allowance for Expected Credit Losses	0
Fair Value	626,170		597,169
States, municipals, and political subdivisions
Schedule of Investments [Line Items]
Amortized Cost	4,143,375		4,415,008
Gross Unrealized Gains	424,024		225,072
Gross Unrealized Losses	(5,033)		(1,230)
Allowance for Expected Credit Losses	0
Fair Value	4,562,366		4,638,850
Corporate securities
Schedule of Investments [Line Items]
Amortized Cost	44,810,937		44,493,799
Gross Unrealized Gains	4,157,060		2,603,636
Gross Unrealized Losses	(450,052)		(288,334)
Allowance for Expected Credit Losses	(81,426)	(51,135)	0
Fair Value	48,436,519		46,809,101
Redeemable preferred stocks
Schedule of Investments [Line Items]
Amortized Cost	66,071		87,237
Gross Unrealized Gains	461		3,677
Gross Unrealized Losses	(859)		(4,249)
Allowance for Expected Credit Losses	0
Fair Value	65,673		86,665
Fixed maturities
Schedule of Investments [Line Items]
Amortized Cost	61,703,564		60,741,093
Gross Unrealized Gains	5,019,654		3,100,992
Gross Unrealized Losses	(518,856)		(325,660)
Allowance for Expected Credit Losses	(82,081)	$ (51,793)	0
Fair Value	66,122,281		63,516,425
Short-term investments
Schedule of Investments [Line Items]
Amortized Cost	1,037,969		1,229,651
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Allowance for Expected Credit Losses	0
Fair Value	$ 1,037,969		$ 1,229,651