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Investment Information - Aging of available for sale securities in an unrealized loss position (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	$ 15,389,158	$ 12,238,683
Estimated Fair Value - 12 Months or More	888,195	297,357
Estimated Fair Value	16,277,353	12,536,040
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	(650,472)	(157,279)
Gross Unrealized Losses - 12 Months or More	(76,180)	(14,265)
Gross Unrealized Losses	(726,652)	(171,544)
Fixed maturities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	14,969,001	11,953,950
Estimated Fair Value - 12 Months or More	888,195	297,357
Estimated Fair Value	15,857,196	12,251,307
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	(649,825)	(156,689)
Gross Unrealized Losses - 12 Months or More	(76,180)	(14,265)
Gross Unrealized Losses	(726,005)	(170,954)
Fixed maturities | Corporate bonds
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	5,798,556	3,639,582
Estimated Fair Value - 12 Months or More	513,866	98,867
Estimated Fair Value	6,312,422	3,738,449
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	(305,661)	(63,938)
Gross Unrealized Losses - 12 Months or More	(50,803)	(5,256)
Gross Unrealized Losses	(356,464)	(69,194)
Fixed maturities | Mortgage backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	328,090	222,176
Estimated Fair Value - 12 Months or More	68,490	46,809
Estimated Fair Value	396,580	268,985
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	(21,714)	(3,545)
Gross Unrealized Losses - 12 Months or More	(6,716)	(1,865)
Gross Unrealized Losses	(28,430)	(5,410)
Fixed maturities | Municipal bonds
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	203,717	26,665
Estimated Fair Value - 12 Months or More	14,759	16,361
Estimated Fair Value	218,476	43,026
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	(9,578)	(385)
Gross Unrealized Losses - 12 Months or More	(1,322)	(1,024)
Gross Unrealized Losses	(10,900)	(1,409)
Fixed maturities | Commercial mortgage backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	942,203	675,603
Estimated Fair Value - 12 Months or More	1,654	5,908
Estimated Fair Value	943,857	681,511
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	(15,815)	(2,805)
Gross Unrealized Losses - 12 Months or More	(47)	(312)
Gross Unrealized Losses	(15,862)	(3,117)
Fixed maturities | US government and government agencies
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	4,201,416	4,211,621
Estimated Fair Value - 12 Months or More	80,735	33,373
Estimated Fair Value	4,282,151	4,244,994
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	(173,754)	(44,180)
Gross Unrealized Losses - 12 Months or More	(5,082)	(1,787)
Gross Unrealized Losses	(178,836)	(45,967)
Fixed maturities | Non-US government securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	2,045,013	1,511,301
Estimated Fair Value - 12 Months or More	98,575	62,957
Estimated Fair Value	2,143,588	1,574,258
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	(82,107)	(31,983)
Gross Unrealized Losses - 12 Months or More	(9,933)	(2,766)
Gross Unrealized Losses	(92,040)	(34,749)
Fixed maturities | Asset backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	1,450,006	1,667,002
Estimated Fair Value - 12 Months or More	110,116	33,082
Estimated Fair Value	1,560,122	1,700,084
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	(41,196)	(9,853)
Gross Unrealized Losses - 12 Months or More	(2,277)	(1,255)
Gross Unrealized Losses	(43,473)	(11,108)
Short-term investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	420,157	284,733
Estimated Fair Value - 12 Months or More	0	0
Estimated Fair Value	420,157	284,733
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	(647)	(590)
Gross Unrealized Losses - 12 Months or More	0	0
Gross Unrealized Losses	$ (647)	$ (590)

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