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Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	$ 17,259,981	$ 15,745,539
Equity securities	12,787,616	11,784,521
Short-term investments, available-for-sale	2,115,757	2,524,910
Total Investments	32,163,354	30,054,970
Fixed Maturity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	17,259,981	15,745,539
U.S. Treasury securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	5,665,523	5,087,858
U.S. government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	1,562,068	1,346,236
Obligations of states, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	3,653,829	3,619,492
Foreign governments, agencies and supranationals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	3,282,500	2,706,523
Agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	2,824,277	2,649,813
Non-agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	67,045	118,030
Corporate and university bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	204,739	217,587
Fair value, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	17,259,981	15,745,539
Equity securities	12,787,616	11,784,521
Short-term investments, available-for-sale	2,115,757	2,524,910
Total Investments	32,163,354	30,054,970
Fair value, recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Equity securities	12,787,616	11,783,137
Short-term investments, available-for-sale	1,942,518	2,363,736
Total Investments	14,730,134	14,146,873
Fair value, recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	17,259,981	15,745,539
Equity securities	0	0
Short-term investments, available-for-sale	173,239	161,174
Total Investments	17,433,220	15,906,713
Fair value, recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Equity securities	0	1,384
Short-term investments, available-for-sale	0	0
Total Investments	0	1,384
Fair value, recurring [Member] | U.S. Treasury securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	5,665,523	5,087,858
Fair value, recurring [Member] | U.S. Treasury securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | U.S. Treasury securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	5,665,523	5,087,858
Fair value, recurring [Member] | U.S. Treasury securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | U.S. government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	1,562,068	1,346,236
Fair value, recurring [Member] | U.S. government-sponsored enterprises [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | U.S. government-sponsored enterprises [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	1,562,068	1,346,236
Fair value, recurring [Member] | U.S. government-sponsored enterprises [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Obligations of states, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	3,653,829	3,619,492
Fair value, recurring [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	3,653,829	3,619,492
Fair value, recurring [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Foreign governments, agencies and supranationals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	3,282,500	2,706,523
Fair value, recurring [Member] | Foreign governments, agencies and supranationals [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Foreign governments, agencies and supranationals [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	3,282,500	2,706,523
Fair value, recurring [Member] | Foreign governments, agencies and supranationals [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	2,824,277	2,649,813
Fair value, recurring [Member] | Agency mortgage-backed securities | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Agency mortgage-backed securities | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	2,824,277	2,649,813
Fair value, recurring [Member] | Agency mortgage-backed securities | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Non-agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	67,045	118,030
Fair value, recurring [Member] | Non-agency mortgage-backed securities | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Non-agency mortgage-backed securities | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	67,045	118,030
Fair value, recurring [Member] | Non-agency mortgage-backed securities | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Corporate and university bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	204,739	217,587
Fair value, recurring [Member] | Corporate and university bonds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Corporate and university bonds [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	204,739	217,587
Fair value, recurring [Member] | Corporate and university bonds [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	0	0
Fair value, recurring [Member] | Insurance, banks and other financial institutions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	5,284,769	4,970,120
Fair value, recurring [Member] | Insurance, banks and other financial institutions [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	5,284,769	4,968,736
Fair value, recurring [Member] | Insurance, banks and other financial institutions [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Fair value, recurring [Member] | Insurance, banks and other financial institutions [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	1,384
Fair value, recurring [Member] | Industrial, consumer and all other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	7,502,847	6,814,401
Fair value, recurring [Member] | Industrial, consumer and all other [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	7,502,847	6,814,401
Fair value, recurring [Member] | Industrial, consumer and all other [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0
Fair value, recurring [Member] | Industrial, consumer and all other [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	$ 0	$ 0

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