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Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	$ 14,519,370	$ 14,372,732
Equity securities	10,570,975	9,577,871
Short-term investments, available-for-sale	2,617,642	2,571,382
Total Investments	27,707,987	26,521,985
U.S. Treasury securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	4,045,990	3,784,713
U.S. government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	1,193,895	1,142,814
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 investments, estimated fair value	3,846,588	4,029,305
Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	1,828,041	1,783,421
Commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	2,268,156	2,243,658
Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	441,678	470,422
Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	895,022	918,399
Fair value, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	14,519,370	14,372,732
Equity securities	10,570,975	9,577,871
Short-term investments, available-for-sale	2,617,642	2,571,382
Total Investments	27,707,987	26,521,985
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 investments, estimated fair value	0	0
Equity securities	10,569,818	9,576,877
Short-term investments, available-for-sale	2,453,881	2,402,099
Total Investments	13,023,699	11,978,976
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 investments, estimated fair value	14,519,370	14,372,732
Equity securities	0	0
Short-term investments, available-for-sale	163,761	169,283
Total Investments	14,683,131	14,542,015
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 investments, estimated fair value	0	0
Equity securities	1,157	994
Short-term investments, available-for-sale	0	0
Total Investments	1,157	994
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 investments, estimated fair value	4,045,990	3,784,713
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 investments, 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 investments, estimated fair value	4,045,990	3,784,713
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 investments, 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 investments, estimated fair value	1,193,895	1,142,814
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 investments, 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 investments, estimated fair value	1,193,895	1,142,814
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 investments, 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 investments, estimated fair value	3,846,588	4,029,305
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 investments, 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 investments, estimated fair value	3,846,588	4,029,305
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 investments, estimated fair value	0	0
Fair value, recurring [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	1,828,041	1,783,421
Fair value, recurring [Member] | Foreign governments [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	0	0
Fair value, recurring [Member] | Foreign governments [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	1,828,041	1,783,421
Fair value, recurring [Member] | Foreign governments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	0	0
Fair value, recurring [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	2,268,156	2,243,658
Fair value, recurring [Member] | Commercial mortgage-backed securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	0	0
Fair value, recurring [Member] | Commercial mortgage-backed securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	2,268,156	2,243,658
Fair value, recurring [Member] | Commercial mortgage-backed securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	0	0
Fair value, recurring [Member] | Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	441,678	470,422
Fair value, recurring [Member] | Residential mortgage-backed securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	0	0
Fair value, recurring [Member] | Residential mortgage-backed securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	441,678	470,422
Fair value, recurring [Member] | Residential mortgage-backed securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	0	0
Fair value, recurring [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	895,022	918,399
Fair value, recurring [Member] | Corporate bonds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	0	0
Fair value, recurring [Member] | Corporate bonds [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, estimated fair value	895,022	918,399
Fair value, recurring [Member] | Corporate bonds [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity investments, 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	4,178,735	3,695,369
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	4,177,578	3,694,375
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	1,157	994
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	6,392,240	5,882,502
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	6,392,240	5,882,502
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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current.

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Sum of the carrying amounts as of the balance sheet date of all investments.

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