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Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | $ 7,814,779 | $ 7,426,555
Equity investments trading | 413,271 | 388,254
Other investments | 738,919 | 594,793
Other assets | 5,259 | 2,542
Other liabilities | (14,208) | (5,494)
Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 7,814,779 | 7,426,555
Short term investments | 2,461,415 | 991,863
Equity investments trading | 413,271 | 388,254
Other investments | 738,919 | 594,793
Other assets |  | 1,684
Other liabilities | (7,355) |
Total assets and liabilities measured on recurring basis | 11,421,029 | 9,403,149
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 | 3,117,911 | 3,168,763
Short term investments | 0 | 0
Equity investments trading | 413,271 | 388,254
Other investments | 0 | 0
Other assets | 249 |
Other liabilities |  | (45)
Total assets and liabilities measured on recurring basis | 3,531,431 | 3,556,972
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 | 4,696,868 | 4,257,792
Short term investments | 2,461,415 | 991,863
Equity investments trading | 0 | 0
Other investments | 525,126 | 380,475
Other assets | 1,345 | 4,681
Total assets and liabilities measured on recurring basis | 7,684,754 | 5,634,811
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 | 0 | 0
Other liabilities | (8,949) | (2,952)
Total assets and liabilities measured on recurring basis | (8,949) | (2,952)
Fair Value, Measurements, Recurring | U.S. treasuries |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 3,117,911 | 3,168,763
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 | 3,117,911 | 3,168,763
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 | 143,980 | 47,646
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 | 143,980 | 47,646
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 | Municipal |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 7,061 | 509,802
Fair Value, Measurements, Recurring | Municipal | 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 | Municipal | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 7,061 | 509,802
Fair Value, Measurements, Recurring | Municipal | 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 (Sovereign debt) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 254,169 | 287,660
Fair Value, Measurements, Recurring | Non-U.S. government (Sovereign debt) | 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 (Sovereign debt) | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturity investments | 254,169 | 287,660
Fair Value, Measurements, Recurring | Non-U.S. government (Sovereign debt) | 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 | 137,512 | 163,651
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 | 137,512 | 163,651
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 | 2,448,795 | 2,063,459
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 | 2,448,795 | 2,063,459
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 | 836,376 | 500,456
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 | 836,376 | 500,456
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 | 289,649 | 300,331
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 | 289,649 | 300,331
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 | 257,434 | 202,062
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 | 257,434 | 202,062
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 | 321,892 | 182,725
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 | 321,892 | 182,725
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 | 525,126 | 380,475
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 | 525,126 | 380,475
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 partnerships |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 185,121 | 196,220
Fair Value, Measurements, Recurring | Private equity partnerships | 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 partnerships | 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 partnerships | 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 | Senior secured bank loan funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other investments | 17,057 | 17,574
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 | 11,615 | 524
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 liabilities | (8,949) | (2,952)
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 |
Other liabilities |  | 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 |
Other liabilities |  | 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 liabilities | (8,949) | (2,952)
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 | 5,300 | 2,500
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 | (14,200) | (5,500)
Fair Value, Measurements, Recurring | Derivatives |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other assets | 1,594 | 4,636
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 assets | 249 |
Other liabilities |  | (45)
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 | 1,345 | 4,681
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 |
Other liabilities |  | $ 0

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