XML 107 R90.htm IDEA: XBRL DOCUMENT
v3.20.4

Fair Value - Fair Value Hierarchy (Details) - Recurring - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | $ 9,280,391 | $ 6,932,402
Liabilities measured at fair value |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Contingent consideration liability |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Securities sold but not yet purchased |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value | [1] | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative instruments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 5,463,356 | 4,805,581
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 5,463,356 | 4,805,581
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,920,565 | 904,804
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,401,653 | 789,596
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivative instruments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 494,817 | 432,421
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Municipal bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  |  | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Commercial mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 164 | 1,852
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Non-US government securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 420,131 | 333,320
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Equity securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 23,373 | 43,962
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Other investments fair value option |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 51,149 | 53,287
Significant Other Observable Inputs (Level 2) |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 15,724,091 | 14,641,044
Liabilities measured at fair value |  | (130,384) | (106,007)
Significant Other Observable Inputs (Level 2) | Contingent consideration liability |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value |  | 0 | 0
Significant Other Observable Inputs (Level 2) | Securities sold but not yet purchased |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value | [1] | (21,679) | (66,257)
Significant Other Observable Inputs (Level 2) | Derivative instruments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value | [2] | (108,705) | (39,750)
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 13,529,813 | 12,443,388
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 7,856,558 | 6,397,740
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 630,001 | 562,055
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 494,522 | 881,926
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 389,900 | 733,108
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 93,721 | 111,011
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 2,433,733 | 2,078,757
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 1,631,378 | 1,678,791
Significant Other Observable Inputs (Level 2) | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 4,357 | 51,742
Significant Other Observable Inputs (Level 2) | Equity securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 17,291 | 4,798
Significant Other Observable Inputs (Level 2) | Derivative instruments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 177,383 | 48,946
Significant Other Observable Inputs (Level 2) | Fair value option |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,995,247 | 2,092,170
Significant Other Observable Inputs (Level 2) | Fair value option | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 650,309 | 487,470
Significant Other Observable Inputs (Level 2) | Fair value option | Mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 3,282 | 11,947
Significant Other Observable Inputs (Level 2) | Fair value option | Municipal bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  |  | 377
Significant Other Observable Inputs (Level 2) | Fair value option | Commercial mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,090 | 1,134
Significant Other Observable Inputs (Level 2) | Fair value option | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 110 | 110
Significant Other Observable Inputs (Level 2) | Fair value option | Non-US government securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 35,263 | 50,465
Significant Other Observable Inputs (Level 2) | Fair value option | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 152,151 | 200,163
Significant Other Observable Inputs (Level 2) | Fair value option | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 136,877 | 43,694
Significant Other Observable Inputs (Level 2) | Fair value option | Equity securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 188 | 641
Significant Other Observable Inputs (Level 2) | Fair value option | Other investments fair value option |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,015,977 | 1,296,169
Significant Unobservable Inputs (Level 3) |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 182,530 | 197,799
Liabilities measured at fair value |  | (461) | (7,998)
Significant Unobservable Inputs (Level 3) | Contingent consideration liability |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value |  | (461) | (7,998)
Significant Unobservable Inputs (Level 3) | Securities sold but not yet purchased |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value | [1] | 0 | 0
Significant Unobservable Inputs (Level 3) | Derivative instruments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value | [2] | 0 | 0
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 3,439 | 14,067
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 13 | 8,851
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 254
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 0
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 0
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 0
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 0 | 0
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 3,426 | 4,962
Significant Unobservable Inputs (Level 3) | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Significant Unobservable Inputs (Level 3) | Equity securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 42,015 | 55,889
Significant Unobservable Inputs (Level 3) | Derivative instruments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 0 | 0
Significant Unobservable Inputs (Level 3) | Fair value option |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 137,076 | 127,843
Significant Unobservable Inputs (Level 3) | Fair value option | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 985 | 932
Significant Unobservable Inputs (Level 3) | Fair value option | Mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Significant Unobservable Inputs (Level 3) | Fair value option | Municipal bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  |  | 0
Significant Unobservable Inputs (Level 3) | Fair value option | Commercial mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Significant Unobservable Inputs (Level 3) | Fair value option | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Significant Unobservable Inputs (Level 3) | Fair value option | Non-US government securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Significant Unobservable Inputs (Level 3) | Fair value option | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Significant Unobservable Inputs (Level 3) | Fair value option | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 | 0
Significant Unobservable Inputs (Level 3) | Fair value option | Equity securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 68,988 | 58,094
Significant Unobservable Inputs (Level 3) | Fair value option | Other investments fair value option |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 67,103 | 68,817
Estimated Fair Value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 26,384,668 | 22,782,288
Liabilities measured at fair value |  | (130,845) | (114,005)
Estimated Fair Value | Contingent consideration liability |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value |  | (461) | (7,998)
Estimated Fair Value | Securities sold but not yet purchased |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value | [1] | (21,679) | (66,257)
Estimated Fair Value | Derivative instruments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value | [2] | (108,705) | (39,750)
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 18,996,608 | 17,263,036
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 7,856,571 | 6,406,591
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 630,001 | 562,309
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 494,522 | 881,926
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 389,900 | 733,108
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 5,557,077 | 4,916,592
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 2,433,733 | 2,078,757
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | 1,634,804 | 1,683,753
Estimated Fair Value | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,924,922 | 956,546
Estimated Fair Value | Equity securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,460,959 | 850,283
Estimated Fair Value | Derivative instruments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 177,383 | 48,946
Estimated Fair Value | Fair value option |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 3,824,796 | 3,663,477
Estimated Fair Value | Fair value option | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 651,294 | 488,402
Estimated Fair Value | Fair value option | Mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 3,282 | 11,947
Estimated Fair Value | Fair value option | Municipal bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  |  | 377
Estimated Fair Value | Fair value option | Commercial mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,090 | 1,134
Estimated Fair Value | Fair value option | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 274 | 1,962
Estimated Fair Value | Fair value option | Non-US government securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 35,263 | 50,465
Estimated Fair Value | Fair value option | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 152,151 | 200,163
Estimated Fair Value | Fair value option | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 557,008 | 377,014
Estimated Fair Value | Fair value option | Equity securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 92,549 | 102,697
Estimated Fair Value | Fair value option | Other investments fair value option |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,134,229 | 1,418,273
Estimated Fair Value | Fair value option | Other investments at net asset value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [4] | $ 1,197,656 | $ 1,011,043
[1] | Represents the Company’s obligations to deliver securities that it did not own at the time of sale. Such amounts are included in “other liabilities” on the Company’s consolidated balance sheets.
[2] | See Note 11
[3] | 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 Note 9
[4] | In accordance with applicable accounting guidance, certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets.

X
- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_AssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Fair value of financial and nonfinancial obligations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByLiabilityClassAxis=acgl_ContingentConsiderationLiabilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByLiabilityClassAxis=us-gaap_SecuritiesSoldNotYetPurchasedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByLiabilityClassAxis=us-gaap_DerivativeFinancialInstrumentsLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_MunicipalBondsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=acgl_NonUsGovernmentSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_DerivativeFinancialInstrumentsAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=acgl_InvestmentsFairValueOptionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=acgl_ShortTermInvestmentsFairValueOptionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=acgl_EquitySecuritiesFairValueOptionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=acgl_OtherInvestmentsFairValueOptionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=acgl_FairValueOptionNetAssetValueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |