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Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	$ 13,506,503	$ 11,171,655
Short term investments	4,993,735	4,566,277
Equity investments trading	702,617	436,931
Other investments	1,256,948	1,087,377
Other assets	8,982	32,944
Other liabilities	(15,193)	(28,213)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	13,506,503	11,171,655
Short term investments	4,993,735	4,566,277
Equity investments trading	702,617	436,931
Other investments	1,256,948	1,087,377
Other assets	16,662	21,668
Total other assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	20,476,465	17,283,908
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	4,960,409	4,467,345
Short term investments	0	0
Equity investments trading	702,617	436,931
Other investments	0	0
Other liabilities	(411)	(1,020)
Total other assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	5,662,615	4,903,256
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	8,546,094	6,704,310
Short term investments	4,993,735	4,566,277
Equity investments trading	0	0
Other investments	881,290	781,641
Other assets	23,284	17,957
Total other assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	14,444,403	12,070,185
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Short term investments	0	0
Equity investments trading	0	0
Other investments	79,807	74,634
Other assets		4,731
Other liabilities	(6,211)
Total other assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	73,596	79,365
Fair Value, Measurements, Recurring | U.S. treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	4,960,409	4,467,345
Fair Value, Measurements, Recurring | U.S. treasuries | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	4,960,409	4,467,345
Fair Value, Measurements, Recurring | U.S. treasuries | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | U.S. treasuries | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	368,032	343,031
Fair Value, Measurements, Recurring | Agencies | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Agencies | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	368,032	343,031
Fair Value, Measurements, Recurring | Agencies | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	491,531	497,392
Fair Value, Measurements, Recurring | Non-U.S. government | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Non-U.S. government | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	491,531	497,392
Fair Value, Measurements, Recurring | Non-U.S. government | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	338,014	321,356
Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	338,014	321,356
Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	4,261,025	3,075,660
Fair Value, Measurements, Recurring | Corporate | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Corporate | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	4,261,025	3,075,660
Fair Value, Measurements, Recurring | Corporate | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	1,113,792	1,148,499
Fair Value, Measurements, Recurring | Agency mortgage-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Agency mortgage-backed | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	1,113,792	1,148,499
Fair Value, Measurements, Recurring | Agency mortgage-backed | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Non-agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	291,444	294,604
Fair Value, Measurements, Recurring | Non-agency mortgage-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Non-agency mortgage-backed | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	291,444	294,604
Fair Value, Measurements, Recurring | Non-agency mortgage-backed | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	791,272	468,698
Fair Value, Measurements, Recurring | Commercial mortgage-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Commercial mortgage-backed | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	791,272	468,698
Fair Value, Measurements, Recurring | Commercial mortgage-backed | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	890,984	555,070
Fair Value, Measurements, Recurring | Asset-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Asset-backed | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	890,984	555,070
Fair Value, Measurements, Recurring | Asset-backed | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Catastrophe bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	881,290	781,641
Fair Value, Measurements, Recurring | Catastrophe bonds | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Catastrophe bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	881,290	781,641
Fair Value, Measurements, Recurring | Catastrophe bonds | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Private equity investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	345,501	271,047
Fair Value, Measurements, Recurring | Private equity investments | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Private equity investments | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Private equity investments | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	79,807	74,634
Fair Value, Measurements, Recurring | Senior secured bank loan funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	19,604	22,598
Fair Value, Measurements, Recurring | Senior secured bank loan funds | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Senior secured bank loan funds | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Senior secured bank loan funds | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	10,553	12,091
Fair Value, Measurements, Recurring | Hedge funds | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Hedge funds | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Hedge funds | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		4,731
Other liabilities	(6,211)
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0	0
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0	0
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets		4,731
Other liabilities	(6,211)
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts | Significant Unobservable Inputs (Level 3) | Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	1,400	32,900
Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts | Significant Unobservable Inputs (Level 3) | Other liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other liabilities	(7,600)	(28,200)
Fair Value, Measurements, Recurring | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	22,873	16,937
Fair Value, Measurements, Recurring | Derivatives | Quoted Prices in Active Markets for Identical  Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other liabilities	(411)	(1,020)
Fair Value, Measurements, Recurring | Derivatives | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	23,284	17,957
Fair Value, Measurements, Recurring | Derivatives | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	$ 0	$ 0

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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