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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.