Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	$ 20,061,584		$ 19,732,825
Gross Unrealized Gains	115,738		198,691
Gross Unrealized Losses	(1,862,287)		(171,544)
Allowance for Expected Credit Losses	(45,993)	$ (58,410)	(2,883)	$ (2,111)	$ (2,124)	$ (2,397)
Cost or Amortized Cost	21,854,126		19,708,561
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	18,120,727		17,998,109
Gross Unrealized Gains	114,319		198,123
Gross Unrealized Losses	(1,860,654)		(170,954)
Allowance for Expected Credit Losses	(45,993)		(2,883)
Cost or Amortized Cost	19,913,055		17,973,823
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	7,096,269		6,553,333
Gross Unrealized Gains	51,777		104,170
Gross Unrealized Losses	(895,365)		(69,194)
Allowance for Expected Credit Losses	(35,657)		(2,037)
Cost or Amortized Cost	7,975,514		6,520,394
Mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	763,371		408,477
Gross Unrealized Gains	7,561		2,825
Gross Unrealized Losses	(84,183)		(5,410)
Allowance for Expected Credit Losses	2		(48)
Cost or Amortized Cost	839,991		411,110
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	407,724		404,666
Gross Unrealized Gains	5,437		18,724
Gross Unrealized Losses	(37,692)		(1,409)
Allowance for Expected Credit Losses	(99)	$ (298)	(2)	$ (2)	$ (6)	$ (11)
Cost or Amortized Cost	440,078		387,353
Commercial mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,064,238		1,046,484
Gross Unrealized Gains	3,079		1,740
Gross Unrealized Losses	(50,010)		(3,117)
Allowance for Expected Credit Losses	(1,583)		(6)
Cost or Amortized Cost	1,112,752		1,047,867
US government and government agencies
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	5,126,407		4,772,764
Gross Unrealized Gains	36,074		10,076
Gross Unrealized Losses	(400,784)		(45,967)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	5,491,117		4,808,655
Non-US government securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,073,170		2,120,294
Gross Unrealized Gains	9,315		54,048
Gross Unrealized Losses	(287,583)		(34,749)
Allowance for Expected Credit Losses	(1,720)		(82)
Cost or Amortized Cost	2,353,158		2,101,077
Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,589,548		2,692,091
Gross Unrealized Gains	1,076		6,540
Gross Unrealized Losses	(105,037)		(11,108)
Allowance for Expected Credit Losses	(6,936)		(708)
Cost or Amortized Cost	1,700,445		2,697,367
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,940,857		1,734,716
Gross Unrealized Gains	1,419		568
Gross Unrealized Losses	(1,633)		(590)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	$ 1,941,071		$ 1,734,738