Fair Value Measurements (Balances Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2026	Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale fixed maturity securities, estimated fair value	$ 17,295,062	$ 17,797,986
Equity securities	13,110,683	13,004,312
Short-term investments, available-for-sale	2,769,092	2,033,662
Total Investments	33,174,837	32,835,960
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	17,295,062	17,797,986
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	5,792,654	5,927,907
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	1,582,349	1,640,234
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	3,375,628	3,672,468
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	3,377,830	3,374,915
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	2,949,749	2,901,120
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	43,262	65,758
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	173,590	215,584
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	17,295,062	17,797,986
Equity securities	13,110,683	13,004,312
Short-term investments, available-for-sale	2,769,092	2,033,662
Total Investments	33,174,837	32,835,960
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	13,110,683	13,004,312
Short-term investments, available-for-sale	2,547,949	1,843,163
Total Investments	15,658,632	14,847,475
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	17,295,062	17,797,986
Equity securities	0	0
Short-term investments, available-for-sale	221,143	190,499
Total Investments	17,516,205	17,988,485
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	0
Short-term investments, available-for-sale	0	0
Total Investments	0	0
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	5,792,654	5,927,907
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	5,792,654	5,927,907
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	1,582,349	1,640,234
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	1,582,349	1,640,234
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	3,375,628	3,672,468
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	3,375,628	3,672,468
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	3,377,830	3,374,915
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	3,377,830	3,374,915
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	2,949,749	2,901,120
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	2,949,749	2,901,120
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	43,262	65,758
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	43,262	65,758
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	173,590	215,584
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	173,590	215,584
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	4,937,280	5,441,617
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	4,937,280	5,441,617
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	0
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	8,173,403	7,562,695
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	8,173,403	7,562,695
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