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Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) | Jun. 30, 2021 | Dec. 31, 2020
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | $ 13,418,389,000 | $ 13,506,503,000
Short term investments | 4,392,652,000 | 4,993,735,000
Equity investments trading | 577,090,000 | 702,617,000
Other investments | 1,585,036,000 | 1,256,948,000
Derivative assets | 9,465,000 | 45,233,000
Derivative liabilities | (15,630,000) | (22,360,000)
Other assets | 4,418,000 | 8,982,000
Other liabilities | (10,611,000) | (15,193,000)
Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 13,418,389,000 | 13,506,503,000
Short term investments | 4,392,652,000 | 4,993,735,000
Equity investments trading | 577,090,000 | 702,617,000
Other investments | 1,585,036,000 | 1,256,948,000
Assumed and ceded (re)insurance contracts | (6,193,000) | (6,211,000)
Derivative assets | 9,465,000 | 45,233,000
Derivative liabilities | (15,630,000) | (22,360,000)
Other assets |  | 16,662,000
Other liabilities | (12,358,000) |
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs | 19,960,809,000 | 20,476,465,000
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 1,107,741,000 | 961,097,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 6,327,895,000 | 4,960,409,000
Short term investments | 0 | 0
Equity investments trading | 577,090,000 | 702,617,000
Other investments | 0 | 0
Assumed and ceded (re)insurance contracts | 0 | 0
Derivative assets | 837,000 | 156,000
Derivative liabilities | (1,281,000) | (567,000)
Other liabilities | (444,000) | (411,000)
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs | 6,904,541,000 | 5,662,615,000
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 7,090,494,000 | 8,546,094,000
Short term investments | 4,392,652,000 | 4,993,735,000
Equity investments trading | 0 | 0
Other investments | 1,026,397,000 | 881,290,000
Assumed and ceded (re)insurance contracts | 0 | 0
Derivative assets | 8,628,000 | 45,077,000
Derivative liabilities | (14,349,000) | (21,793,000)
Other assets |  | 23,284,000
Other liabilities | (5,721,000) |
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs | 12,503,822,000 | 14,444,403,000
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
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 | 81,344,000 | 79,807,000
Assumed and ceded (re)insurance contracts | (6,193,000) | (6,211,000)
Derivative assets | 0 | 0
Derivative liabilities | 0 | 0
Other liabilities | (6,193,000) | (6,211,000)
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs | 75,151,000 | 73,596,000
Significant Unobservable Inputs (Level 3) | Other assets | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assumed and ceded (re)insurance contracts | 0 | (1,400,000)
Significant Unobservable Inputs (Level 3) | Other liabilities | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assumed and ceded (re)insurance contracts | (6,200,000) | 7,600,000
U.S. treasuries | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 6,327,895,000 | 4,960,409,000
U.S. treasuries | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 6,327,895,000 | 4,960,409,000
U.S. treasuries | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
U.S. treasuries | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Agencies | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 325,051,000 | 368,032,000
Agencies | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Agencies | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 325,051,000 | 368,032,000
Agencies | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Non-U.S. government | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 508,320,000 | 491,531,000
Non-U.S. government | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Non-U.S. government | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 508,320,000 | 491,531,000
Non-U.S. government | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Non-U.S. government-backed corporate | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 396,966,000 | 338,014,000
Non-U.S. government-backed corporate | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Non-U.S. government-backed corporate | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 396,966,000 | 338,014,000
Non-U.S. government-backed corporate | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Corporate | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 3,387,433,000 | 4,261,025,000
Corporate | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Corporate | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 3,387,433,000 | 4,261,025,000
Corporate | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Agency mortgage-backed | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 703,757,000 | 1,113,792,000
Agency mortgage-backed | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Agency mortgage-backed | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 703,757,000 | 1,113,792,000
Agency mortgage-backed | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Non-agency mortgage-backed | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 260,432,000 | 291,444,000
Non-agency mortgage-backed | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Non-agency mortgage-backed | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 260,432,000 | 291,444,000
Non-agency mortgage-backed | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Commercial mortgage-backed | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 588,262,000 | 791,272,000
Commercial mortgage-backed | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Commercial mortgage-backed | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 588,262,000 | 791,272,000
Commercial mortgage-backed | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Asset-backed | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 920,273,000 | 890,984,000
Asset-backed | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Asset-backed | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 920,273,000 | 890,984,000
Asset-backed | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 0 | 0
Catastrophe bonds | Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 1,026,397,000 | 881,290,000
Catastrophe bonds | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Catastrophe bonds | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 1,026,397,000 | 881,290,000
Catastrophe bonds | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Private equity and private credit funds | Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 81,344,000 | 79,807,000
Private equity and private credit funds | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Private equity and private credit funds | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Private equity and private credit funds | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 81,344,000 | 79,807,000
Fund investments | Fair Value Measured at Net Asset Value Per Share | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | $ 477,295,000 | $ 295,851,000

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