Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Available for sale securities:
Estimated Fair Value		$ 16,841,571		$ 16,894,526
Estimated Fair Value		944,531		956,546
Estimated Fair Value		17,963,544		18,219,582
Gross Unrealized Gains		447,088		351,114
Gross Unrealized Losses		(350,788)		(55,003)
Allowance for Expected Credit Losses		(9,938)		0
Cost or Amortized Cost		16,757,086		16,598,808
Cost or Amortized Cost		944,878		957,283
Cost or Amortized Cost		17,877,182		17,923,471
Fixed maturities
Available for sale securities:
Estimated Fair Value	[1]	17,019,013		17,263,036
Gross Unrealized Gains	[2]	444,627		350,303
Gross Unrealized Losses	[2]	(348,009)		(53,455)
Allowance for Expected Credit Losses	[2],[3]	(9,909)
Cost or Amortized Cost	[1],[2]	16,932,304		16,966,188
Fixed maturities | Corporate bonds
Available for sale securities:
Estimated Fair Value		6,552,825		6,406,591
Gross Unrealized Gains	[2]	176,246		191,889
Gross Unrealized Losses	[2]	(118,340)		(12,793)
Allowance for Expected Credit Losses	[2],[3]	(7,233)
Cost or Amortized Cost	[2]	6,502,152		6,227,495
Fixed maturities | Mortgage backed securities
Available for sale securities:
Estimated Fair Value		466,045		562,309
Gross Unrealized Gains	[2]	16,006		9,669
Gross Unrealized Losses	[2]	(6,208)		(931)
Allowance for Expected Credit Losses	[2],[3]	(208)
Cost or Amortized Cost	[2]	456,455		553,571
Fixed maturities | Municipal bonds
Available for sale securities:
Estimated Fair Value		862,415		881,926
Gross Unrealized Gains	[2]	27,287		24,628
Gross Unrealized Losses	[2]	(7,550)		(2,213)
Allowance for Expected Credit Losses	[2],[3]	(23)
Cost or Amortized Cost	[2]	842,701		859,511
Fixed maturities | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value		780,358		733,108
Gross Unrealized Gains	[2]	11,657		14,951
Gross Unrealized Losses	[2]	(18,410)		(2,330)
Allowance for Expected Credit Losses	[2],[3]	(306)
Cost or Amortized Cost	[2]	787,417		720,487
Fixed maturities | US government and government agencies
Available for sale securities:
Estimated Fair Value		4,574,630		4,916,592
Gross Unrealized Gains	[2]	143,987		36,600
Gross Unrealized Losses	[2]	(300)		(10,134)
Allowance for Expected Credit Losses	[2],[3]	0
Cost or Amortized Cost	[2]	4,430,943		4,890,126
Fixed maturities | Non-US government securities
Available for sale securities:
Estimated Fair Value		2,035,459		2,078,757
Gross Unrealized Gains	[2]	57,289		48,549
Gross Unrealized Losses	[2]	(105,533)		(20,330)
Allowance for Expected Credit Losses	[2],[3]	0
Cost or Amortized Cost	[2]	2,083,703		2,050,538
Fixed maturities | Asset backed securities
Available for sale securities:
Estimated Fair Value		1,747,281		1,683,753
Gross Unrealized Gains	[2]	12,155		24,017
Gross Unrealized Losses	[2]	(91,668)		(4,724)
Allowance for Expected Credit Losses	[2],[3]	(2,139)
Cost or Amortized Cost	[2]	1,828,933		1,664,460
Short-term investments
Available for sale securities:
Estimated Fair Value		944,531		956,546
Gross Unrealized Gains		2,461		811
Gross Unrealized Losses		(2,779)		(1,548)
Allowance for Expected Credit Losses		(29)	[3]	0
Cost or Amortized Cost		$ 944,878		$ 957,283

[1] In securities lending transactions, the Company receives collateral in excess of the fair value of the securities 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] 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.”

[3] 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.