Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands		Jun. 30, 2020		Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		$ 17,207,731			$ 16,894,526
Estimated Fair Value		2,277,866			956,546
Estimated Fair Value		19,943,503			18,219,582
Gross Unrealized Gains		620,486			351,114
Gross Unrealized Losses		(135,183)			(55,003)
Allowance for expected credit losses		(5,869)		$ (9,938)	0
Amortized cost		16,733,493			16,598,808
Cost		2,276,413			957,283
Cost or Amortized Cost		19,464,069			17,923,471
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	[1]	17,665,637			17,263,036
Gross Unrealized Gains	[1]	618,399			350,303
Gross Unrealized Losses	[1]	(134,549)			(53,455)
Allowance for expected credit losses	[1],[2]	(5,869)
Amortized cost	[1]	17,187,656			16,966,188
Fixed maturities | Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		7,143,600			6,406,591
Gross Unrealized Gains	[1]	405,579			191,889
Gross Unrealized Losses	[1]	(34,700)			(12,793)
Allowance for expected credit losses	[1],[2]	(4,116)
Amortized cost	[1]	6,776,837			6,227,495
Fixed maturities | Mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		610,839			562,309
Gross Unrealized Gains	[1]	12,417			9,669
Gross Unrealized Losses	[1]	(5,245)			(931)
Allowance for expected credit losses	[1],[2]	(222)
Amortized cost	[1]	603,889			553,571
Fixed maturities | Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		517,089			881,926
Gross Unrealized Gains	[1]	25,222			24,628
Gross Unrealized Losses	[1]	(542)			(2,213)
Allowance for expected credit losses	[1],[2]	(28)
Amortized cost	[1]	492,437			859,511
Fixed maturities | Commercial mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		395,682			733,108
Gross Unrealized Gains	[1]	7,603			14,951
Gross Unrealized Losses	[1]	(8,005)			(2,330)
Allowance for expected credit losses	[1],[2]	(109)
Amortized cost	[1]	396,193			720,487
Fixed maturities | US government and government agencies
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		5,192,991			4,916,592
Gross Unrealized Gains	[1]	64,502			36,600
Gross Unrealized Losses	[1]	(2,708)			(10,134)
Allowance for expected credit losses	[1],[2]	0
Amortized cost	[1]	5,131,197			4,890,126
Fixed maturities | Non-US government securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		2,171,955			2,078,757
Gross Unrealized Gains	[1]	71,666			48,549
Gross Unrealized Losses	[1]	(43,928)			(20,330)
Allowance for expected credit losses	[1],[2]	0
Amortized cost	[1]	2,144,217			2,050,538
Fixed maturities | Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		1,633,481			1,683,753
Gross Unrealized Gains	[1]	31,410			24,017
Gross Unrealized Losses	[1]	(39,421)			(4,724)
Allowance for expected credit losses	[1],[2]	(1,394)
Amortized cost	[1]	1,642,886			1,664,460
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		2,277,866			956,546
Gross Unrealized Gains		2,087			811
Gross Unrealized Losses		(634)			(1,548)
Allowance for expected credit losses		0	[2]	$ (29)	0
Cost		$ 2,276,413			$ 957,283

[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.