Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	$ 19,812,903		$ 19,732,825
Gross Unrealized Gains	86,856		198,691
Gross Unrealized Losses	(1,258,588)		(171,544)
Allowance for Expected Credit Losses	(58,410)	$ (34,145)	(2,883)	$ (2,124)	$ (3,830)	$ (2,397)
Cost or Amortized Cost	21,043,045		19,708,561
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Allowance for Expected Credit Losses	(298)	$ (106)	(2)	$ (6)	$ (2)	$ (11)
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	17,585,029		17,998,109
Gross Unrealized Gains	84,398		198,123
Gross Unrealized Losses	(1,257,331)		(170,954)
Allowance for Expected Credit Losses	(58,410)		(2,883)
Cost or Amortized Cost	18,816,372		17,973,823
Fixed maturities | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	7,288,555		6,553,333
Gross Unrealized Gains	32,021		104,170
Gross Unrealized Losses	(655,664)		(69,194)
Allowance for Expected Credit Losses	(38,494)		(2,037)
Cost or Amortized Cost	7,950,692		6,520,394
Fixed maturities | Mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	698,168		408,477
Gross Unrealized Gains	4,427		2,825
Gross Unrealized Losses	(55,898)		(5,410)
Allowance for Expected Credit Losses	0		(48)
Cost or Amortized Cost	749,639		411,110
Fixed maturities | Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	417,380		404,666
Gross Unrealized Gains	4,129		18,724
Gross Unrealized Losses	(18,936)		(1,409)
Allowance for Expected Credit Losses	(298)		(2)
Cost or Amortized Cost	432,485		387,353
Fixed maturities | Commercial mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,023,088		1,046,484
Gross Unrealized Gains	1,533		1,740
Gross Unrealized Losses	(42,056)		(3,117)
Allowance for Expected Credit Losses	(3,594)		(6)
Cost or Amortized Cost	1,067,205		1,047,867
Fixed maturities | US government and government agencies
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	4,368,150		4,772,764
Gross Unrealized Gains	25,112		10,076
Gross Unrealized Losses	(240,786)		(45,967)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	4,583,824		4,808,655
Fixed maturities | Non-US government securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,181,943		2,120,294
Gross Unrealized Gains	16,175		54,048
Gross Unrealized Losses	(163,724)		(34,749)
Allowance for Expected Credit Losses	(1,335)		(82)
Cost or Amortized Cost	2,330,827		2,101,077
Fixed maturities | Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,607,745		2,692,091
Gross Unrealized Gains	1,001		6,540
Gross Unrealized Losses	(80,267)		(11,108)
Allowance for Expected Credit Losses	(14,689)		(708)
Cost or Amortized Cost	1,701,700		2,697,367
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,227,874		1,734,716
Gross Unrealized Gains	2,458		568
Gross Unrealized Losses	(1,257)		(590)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	$ 2,226,673		$ 1,734,738