Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2021		Mar. 31, 2021	Dec. 31, 2020		Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		$ 18,073,779			$ 18,717,825
Estimated Fair Value		2,248,613			1,924,922
Estimated Fair Value		20,478,448			20,921,530
Gross Unrealized Gains		390,212			657,620
Gross Unrealized Losses		(78,417)			(79,654)
Allowance for expected credit losses		(2,124)		$ (3,830)	(2,397)		$ (5,869)	$ (9,938)	$ 0
Amortized cost		17,764,783			18,143,305
Cost		2,248,615			1,924,292
Cost or Amortized Cost		$ 20,168,777			20,345,961
Accounting Standards Update		us-gaap:AccountingStandardsUpdate201613Member
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	[1]	$ 18,229,835			18,996,608
Gross Unrealized Gains	[1]	388,808			654,927
Gross Unrealized Losses	[1]	(77,011)			(77,591)
Allowance for expected credit losses	[2]	(2,124)	[1]		(2,397)
Amortized cost	[1]	17,920,162			18,421,669
Fixed maturities | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		7,129,768			7,856,571
Gross Unrealized Gains	[1]	217,061			414,247
Gross Unrealized Losses	[1]	(34,751)			(34,388)
Allowance for expected credit losses	[2]	(1,232)	[1]		(896)
Amortized cost	[1]	6,948,690			7,477,608
Fixed maturities | Mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		389,041			630,001
Gross Unrealized Gains	[1]	4,480			8,939
Gross Unrealized Losses	[1]	(3,479)			(5,028)
Allowance for expected credit losses	[2]	(269)	[1]		(278)
Amortized cost	[1]	388,309			626,368
Fixed maturities | Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		415,483			494,522
Gross Unrealized Gains	[1]	23,153			27,291
Gross Unrealized Losses	[1]	(1,660)			(3,835)
Allowance for expected credit losses	[2]	(6)	[1]		(11)
Amortized cost	[1]	393,996			471,077
Fixed maturities | Commercial mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		266,733			389,900
Gross Unrealized Gains	[1]	4,017			8,722
Gross Unrealized Losses	[1]	(611)			(2,954)
Allowance for expected credit losses	[2]	(3)	[1]		(122)
Amortized cost	[1]	263,330			384,254
Fixed maturities | US government and government agencies
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		5,091,183			5,557,077
Gross Unrealized Gains	[1]	14,814			22,612
Gross Unrealized Losses	[1]	(14,013)			(12,611)
Allowance for expected credit losses	[2]	0	[1]		0
Amortized cost	[1]	5,090,382			5,547,076
Fixed maturities | Non-US government securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		2,449,782			2,433,733
Gross Unrealized Gains	[1]	108,823			153,891
Gross Unrealized Losses	[1]	(18,441)			(8,060)
Allowance for expected credit losses	[2]	(128)	[1]		0
Amortized cost	[1]	2,359,528			2,287,902
Fixed maturities | Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		2,487,845			1,634,804
Gross Unrealized Gains	[1]	16,460			19,225
Gross Unrealized Losses	[1]	(4,056)			(10,715)
Allowance for expected credit losses	[2]	(486)	[1]		(1,090)
Amortized cost	[1]	2,475,927			1,627,384
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		2,248,613			1,924,922
Gross Unrealized Gains		1,404			2,693
Gross Unrealized Losses		(1,406)			(2,063)
Allowance for expected credit losses		0	[2]	$ 0	0	[2]	$ 0	$ (29)	$ 0
Cost		$ 2,248,615			$ 1,924,292

[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.”

[2] Effective January 1, 2020, the Company adopted ASU 2016-13 and as a result any credit impairment losses on the Company’s available-for-sale investments are recorded as an allowance, subject to reversal.