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Investment Information - Aging of available for sale securities in an unrealized loss position (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months		$ 9,415,006	$ 3,788,763
Estimated Fair Value - 12 Months or More		174,082	70,404
Estimated Fair Value		9,589,088	3,859,167
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months		(106,319)	(76,067)
Gross Unrealized Losses - 12 Months or More		(8,798)	(3,587)
Gross Unrealized Losses		(115,117)	(79,654)
Fixed maturities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	9,149,995	3,690,843
Estimated Fair Value - 12 Months or More	[1]	174,082	70,404
Estimated Fair Value	[1]	9,324,077	3,761,247
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(104,539)	(74,004)
Gross Unrealized Losses - 12 Months or More	[1]	(8,798)	(3,587)
Gross Unrealized Losses	[1]	(113,337)	(77,591)
Fixed maturities | Corporate bonds
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	2,744,615	747,442
Estimated Fair Value - 12 Months or More	[1]	68,450	3,934
Estimated Fair Value	[1]	2,813,065	751,376
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(36,595)	(33,086)
Gross Unrealized Losses - 12 Months or More	[1]	(4,054)	(1,302)
Gross Unrealized Losses	[1]	(40,649)	(34,388)
Fixed maturities | Mortgage backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	257,789	284,619
Estimated Fair Value - 12 Months or More	[1]	22,769	3,637
Estimated Fair Value	[1]	280,558	288,256
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(3,113)	(4,788)
Gross Unrealized Losses - 12 Months or More	[1]	(853)	(240)
Gross Unrealized Losses	[1]	(3,966)	(5,028)
Fixed maturities | Municipal bonds
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months	[1]	46,450	67,937
Estimated Fair Value - 12 Months or More	[1]	7,148	0
Estimated Fair Value	[1]	53,598	67,937
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(731)	(3,835)
Gross Unrealized Losses - 12 Months or More	[1]	(475)	0
Gross Unrealized Losses	[1]	(1,206)	(3,835)
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	[1]	76,518	126,624
Estimated Fair Value - 12 Months or More	[1]	6,578	2,655
Estimated Fair Value	[1]	83,096	129,279
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(256)	(2,916)
Gross Unrealized Losses - 12 Months or More	[1]	(292)	(38)
Gross Unrealized Losses	[1]	(548)	(2,954)
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	[1]	3,621,719	1,285,907
Estimated Fair Value - 12 Months or More	[1]	9,755	0
Estimated Fair Value	[1]	3,631,474	1,285,907
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(29,797)	(12,611)
Gross Unrealized Losses - 12 Months or More	[1]	(523)	0
Gross Unrealized Losses	[1]	(30,320)	(12,611)
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	[1]	1,283,266	543,844
Estimated Fair Value - 12 Months or More	[1]	22,304	2,441
Estimated Fair Value	[1]	1,305,570	546,285
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(27,523)	(7,658)
Gross Unrealized Losses - 12 Months or More	[1]	(1,425)	(402)
Gross Unrealized Losses	[1]	(28,948)	(8,060)
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]	1,119,638	634,470
Estimated Fair Value - 12 Months or More	[1]	37,078	57,737
Estimated Fair Value	[1]	1,156,716	692,207
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months	[1]	(6,524)	(9,110)
Gross Unrealized Losses - 12 Months or More	[1]	(1,176)	(1,605)
Gross Unrealized Losses	[1]	(7,700)	(10,715)
Short-term investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Estimated Fair Value [Abstract]
Estimated Fair Value - Less than 12 Months		265,011	97,920
Estimated Fair Value - 12 Months or More		0	0
Estimated Fair Value		265,011	97,920
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Gross Unrealized Losses - Less than 12 Months		(1,780)	(2,063)
Gross Unrealized Losses - 12 Months or More		0	0
Gross Unrealized Losses		$ (1,780)	$ (2,063)
[1] In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.” In September 2021, the Company terminated its securities lending program and no longer enters into securities lending agreements with financial institutions.

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