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Fair Value Measurement (Narrative) (Details) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020 | May 05, 2021
Credit Derivatives [Line Items] |  |  |  |  |  |
Goodwill impairment | $ 0 | $ 161,115,000 | $ 0 | $ 161,115,000 | $ 161,115,000 |
Fair value of funded deferred compensation assets | 38,500,000 |  | 38,500,000 |  | 30,600,000 |
Deferred compensation liabilities | 51,456,000 |  | 51,456,000 |  | 30,334,000 |
Fair Value |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Deferred compensation liabilities | 51,500,000 |  | 51,500,000 |  | 30,300,000 |
Fair Value, Nonrecurring |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Fair value, net asset (liability) | 0 |  | 0 |  | 0 |
Specialty P&C |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Goodwill impairment |  | $ 161,100,000 | 0 |  | $ 161,115,000 |
NORCAL Group |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Contingent consideration, maximum estimate |  |  |  |  |  | $ 84,000,000
Fair value of funded deferred compensation assets | 5,100,000 |  | 5,100,000 |  |  |
NORCAL Group | Fair Value |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Deferred compensation liabilities | 18,000,000 |  | $ 18,000,000 |  |  |
Corporate debt, limited observable inputs | NRSRO, Rating BBB |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Credit quality percentage |  |  | 100.00% |  |  |
Corporate debt, limited observable inputs | NRSRO, Rating BB plus |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Credit quality percentage |  |  |  |  | 100.00% |
Other asset-backed securities | NRSRO, Rating AA Minus |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Credit quality percentage |  |  |  |  | 51.00% |
Other asset-backed securities | NRSRO, Rating A |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Credit quality percentage |  |  | 95.00% |  |  |
Mortgages | LIBOR |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Basis spread on variable rate for debt |  |  | 1.325% |  | 1.325% |
Interest Rate Cap |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Premium paid on derivative for right to receive cash payments | $ 2,000,000 |  | $ 2,000,000 |  |  |
Floor interest rate on derivative | 2.35% |  | 2.35% |  |  |
Interest Rate Cap | Other assets |  |  |  |  |  |
Credit Derivatives [Line Items] |  |  |  |  |  |
Derivative asset, notional amount | $ 35,000,000 |  | $ 35,000,000 |  |  |

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