Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021		Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		$ 18,723,035		$ 18,717,825
Estimated Fair Value		1,269,631		1,924,922
Estimated Fair Value		20,122,818		20,921,530
Gross Unrealized Gains		435,320		657,620
Gross Unrealized Losses		(157,036)		(79,654)
Allowance for expected credit losses		(3,830)		(2,397)	$ 0
Amortized cost		18,447,720		18,143,305
Cost		1,269,312		1,924,292
Cost or Amortized Cost		$ 19,848,364		20,345,961
Accounting Standards Update		us-gaap:AccountingStandardsUpdate201613Member
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	[1]	$ 18,853,187		18,996,608
Gross Unrealized Gains	[1]	434,400		654,927
Gross Unrealized Losses	[1]	(156,435)		(77,591)
Allowance for expected credit losses		(3,830)	[1],[2]	(2,397)
Amortized cost	[1]	18,579,052		18,421,669
Fixed maturities | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		8,072,883		7,856,571
Gross Unrealized Gains	[1]	242,832		414,247
Gross Unrealized Losses	[1]	(83,961)		(34,388)
Allowance for expected credit losses		(2,569)	[1],[2]	(896)
Amortized cost	[1]	7,916,581		7,477,608
Fixed maturities | Mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		571,071		630,001
Gross Unrealized Gains	[1]	7,066		8,939
Gross Unrealized Losses	[1]	(11,165)		(5,028)
Allowance for expected credit losses		(325)	[1],[2]	(278)
Amortized cost	[1]	575,495		626,368
Fixed maturities | Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		457,329		494,522
Gross Unrealized Gains	[1]	19,942		27,291
Gross Unrealized Losses	[1]	(4,814)		(3,835)
Allowance for expected credit losses		(2)	[1],[2]	(11)
Amortized cost	[1]	442,203		471,077
Fixed maturities | Commercial mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		255,373		389,900
Gross Unrealized Gains	[1]	2,755		8,722
Gross Unrealized Losses	[1]	(1,721)		(2,954)
Allowance for expected credit losses		(5)	[1],[2]	(122)
Amortized cost	[1]	254,344		384,254
Fixed maturities | US government and government agencies
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		5,042,208		5,557,077
Gross Unrealized Gains	[1]	15,102		22,612
Gross Unrealized Losses	[1]	(30,545)		(12,611)
Allowance for expected credit losses		0	[1],[2]	0
Amortized cost	[1]	5,057,651		5,547,076
Fixed maturities | Non-US government securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		2,425,882		2,433,733
Gross Unrealized Gains	[1]	127,886		153,891
Gross Unrealized Losses	[1]	(19,267)		(8,060)
Allowance for expected credit losses		(51)	[1],[2]	0
Amortized cost	[1]	2,317,314		2,287,902
Fixed maturities | Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		2,028,441		1,634,804
Gross Unrealized Gains	[1]	18,817		19,225
Gross Unrealized Losses	[1]	(4,962)		(10,715)
Allowance for expected credit losses		(878)	[1],[2]	(1,090)
Amortized cost	[1]	2,015,464		1,627,384
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		1,269,631		1,924,922
Gross Unrealized Gains		920		2,693
Gross Unrealized Losses		(601)		(2,063)
Allowance for expected credit losses		0	[2]	0
Cost		$ 1,269,312		$ 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.