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Fair Value - Fair Value Hierarchy (Details) - Recurring - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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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 |  | $ 8,273,826 | $ 9,280,391
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 |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Residential mortgage loans |  |  |
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) | Fixed maturities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 4,431,935 | 5,463,356
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Corporate bonds |  |  |
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) | Fixed maturities | Mortgage backed securities |  |  |
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) | Fixed maturities | Municipal bonds |  |  |
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) | Fixed maturities | Commercial mortgage backed securities |  |  |
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) | Fixed maturities | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 4,431,935 | 5,463,356
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Non-US government securities |  |  |
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) | Fixed maturities | Asset backed securities |  |  |
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) | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 2,046,391 | 1,920,565
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 |  | 1,756,462 | 1,401,653
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 |  | 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 |  | 39,038 | 494,817
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 | 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 |  | 0 | 164
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 |  | 1,190 | 420,131
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 |  | 19,987 | 23,373
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 |  | 17,861 | 51,149
Significant Other Observable Inputs (Level 2) |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 14,325,691 | 15,724,091
Liabilities measured at fair value |  | (43,526) | (130,384)
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] | 0 | (21,679)
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 |  | (43,526) | (108,705)
Significant Other Observable Inputs (Level 2) | Residential mortgage loans |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 7,701 |
Significant Other Observable Inputs (Level 2) | Fixed maturities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 12,332,971 | 13,529,813
Significant Other Observable Inputs (Level 2) | Fixed maturities | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 6,403,604 | 7,856,558
Significant Other Observable Inputs (Level 2) | Fixed maturities | Mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 405,797 | 630,001
Significant Other Observable Inputs (Level 2) | Fixed maturities | Municipal bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 382,722 | 494,522
Significant Other Observable Inputs (Level 2) | Fixed maturities | Commercial mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 579,424 | 389,900
Significant Other Observable Inputs (Level 2) | Fixed maturities | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 28,580 | 93,721
Significant Other Observable Inputs (Level 2) | Fixed maturities | Non-US government securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 1,863,734 | 2,433,733
Significant Other Observable Inputs (Level 2) | Fixed maturities | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 2,669,110 | 1,631,378
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 |  | 1,023,574 | 4,357
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 |  | 31,596 | 17,291
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 |  | 89,958 | 177,383
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 |  | 839,891 | 1,995,247
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 |  | 374,326 | 650,309
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 |  | 0 | 3,282
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 |  | 0 | 1,090
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 |  | 0 | 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 |  | 19,829 | 35,263
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 |  | 19,852 | 152,151
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 |  | 114,491 | 136,877
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 |  | 0 | 188
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 |  | 311,393 | 1,015,977
Significant Unobservable Inputs (Level 3) |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 40,331 | 182,530
Liabilities measured at fair value |  | (17,811) | (461)
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 |  | (17,811) | (461)
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 |  | 0 | 0
Significant Unobservable Inputs (Level 3) | Residential mortgage loans |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 0 |
Significant Unobservable Inputs (Level 3) | Fixed maturities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 3,457 | 3,439
Significant Unobservable Inputs (Level 3) | Fixed maturities | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 13 | 13
Significant Unobservable Inputs (Level 3) | Fixed maturities | Mortgage backed securities |  |  |
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) | Fixed maturities | Municipal bonds |  |  |
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) | Fixed maturities | Commercial mortgage backed securities |  |  |
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) | Fixed maturities | US government and government agencies |  |  |
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) | Fixed maturities | Non-US government securities |  |  |
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) | Fixed maturities | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [2] | 3,444 | 3,426
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 |  | 2,582 | 42,015
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 |  | 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 |  | 34,292 | 137,076
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 |  | 0 | 985
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 | 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 |  | 4,535 | 68,988
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 |  | 29,757 | 67,103
Estimated Fair Value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 23,770,596 | 26,384,668
Liabilities measured at fair value |  | (61,337) | (130,845)
Estimated Fair Value | Contingent consideration liability |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value |  | (17,811) | (461)
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] | 0 | (21,679)
Estimated Fair Value | Derivative instruments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities measured at fair value |  | (43,526) | (108,705)
Estimated Fair Value | Residential mortgage loans |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 7,701 |
Estimated Fair Value | Fixed maturities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 16,768,363 | 18,996,608
Estimated Fair Value | Fixed maturities | Corporate bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 6,403,617 | 7,856,571
Estimated Fair Value | Fixed maturities | Mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 405,797 | 630,001
Estimated Fair Value | Fixed maturities | Municipal bonds |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 382,722 | 494,522
Estimated Fair Value | Fixed maturities | Commercial mortgage backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 579,424 | 389,900
Estimated Fair Value | Fixed maturities | US government and government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 4,460,515 | 5,557,077
Estimated Fair Value | Fixed maturities | Non-US government securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,863,734 | 2,433,733
Estimated Fair Value | Fixed maturities | Asset backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 2,672,554 | 1,634,804
Estimated Fair Value | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 3,069,965 | 1,924,922
Estimated Fair Value | Equity securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 1,790,640 | 1,460,959
Estimated Fair Value | Derivative instruments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 89,958 | 177,383
Estimated Fair Value | Fair value option |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value |  | 2,043,969 | 3,824,796
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 |  | 374,326 | 651,294
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 |  | 0 | 3,282
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 |  | 0 | 1,090
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 |  | 0 | 274
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 |  | 19,829 | 35,263
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 |  | 19,852 | 152,151
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 |  | 115,681 | 557,008
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 |  | 24,522 | 92,549
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 |  | 359,011 | 1,134,229
Estimated Fair Value | Fair value option | Other investments measured at net asset value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets measured at fair value | [3] | $ 1,130,748 | $ 1,197,656
[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] | 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 8, “—Securities Lending Agreements.” In September 2021, the Company terminated its securities lending program and no longer enters into securities lending agreements with financial institutions.
[3] | 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.

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