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Fair Value Measurements (Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis Valuation Techniques) (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	$ 3,549,810,000	$ 3,093,275,000
Equity securities (cost $695,150; $648,282)	803,851,000	724,751,000
Short-term investments (cost $376,547; $494,060)	375,609,000	494,135,000
Equity Contract [Member]
Fair Value Measurement [Line Items]
Derivative liabilitly	0	77,000
Fair Value, Measurements, Recurring [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	3,549,810,000	3,093,275,000
Equity securities (cost $695,150; $648,282)	803,851,000	724,751,000
Short-term investments (cost $376,547; $494,060)	375,609,000	494,135,000
Other assets:	5,725,000	5,665,000
Total assets at fair value	4,734,995,000	4,317,826,000
Liabilities fair value		77,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	13,816,000	22,637,000
Equity securities (cost $695,150; $648,282)	691,782,000	586,367,000
Short-term investments (cost $376,547; $494,060)	370,680,000	464,055,000
Other assets:	0	0
Total assets at fair value	1,076,278,000	1,073,059,000
Liabilities fair value		77,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	3,535,994,000	3,070,638,000
Equity securities (cost $695,150; $648,282)	32,660,000	49,708,000
Short-term investments (cost $376,547; $494,060)	4,929,000	30,080,000
Other assets:	5,725,000	5,665,000
Total assets at fair value	3,579,308,000	3,156,091,000
Liabilities fair value		0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Equity securities (cost $695,150; $648,282)	0	1,203,000
Short-term investments (cost $376,547; $494,060)	0	0
Other assets:	0	0
Total assets at fair value	0	1,203,000
Liabilities fair value		0
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	13,816,000	22,637,000
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	13,816,000	22,637,000
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	2,791,212,000	2,554,208,000
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	2,791,212,000	2,554,208,000
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	93,264,000	63,003,000
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	93,264,000	63,003,000
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	241,366,000	235,565,000
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	241,366,000	235,565,000
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	256,891,000	199,217,000
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	256,891,000	199,217,000
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	153,261,000	18,645,000
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	153,261,000	18,645,000
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Fixed maturity securities (amortized cost $3,388,418; $2,973,276)	0	0
Fair Value, Measurements, Recurring [Member] | Common Stock [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)	691,782,000	586,367,000
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)	691,782,000	586,367,000
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)	0	0
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)	0	0
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)	32,660,000	49,708,000
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)	0	0
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)	32,660,000	49,708,000
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)	0	0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)		1,203,000
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)		0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)		0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)		1,203,000
Fair Value, Measurements, Recurring [Member] | Private Equity Funds Net Asset Value [Member]
Fair Value Measurement [Line Items]
Equity securities (cost $695,150; $648,282)	79,409,000	87,473,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	26,928,000	32,902,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	21,999,000	2,822,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	4,929,000	30,080,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	0	0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	348,676,000	461,233,000
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	348,676,000	461,233,000
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	0	0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	0	0
Fair Value, Measurements, Recurring [Member] | Other
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	5,000
Fair Value, Measurements, Recurring [Member] | Other | Level 1 [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	5,000
Fair Value, Measurements, Recurring [Member] | Other | Level 2 [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	0
Fair Value, Measurements, Recurring [Member] | Other | Level 3 [Member]
Fair Value Measurement [Line Items]
Short-term investments (cost $376,547; $494,060)	$ 0
Fair Value, Measurements, Recurring [Member] | Equity Contract [Member] | Level 1 [Member]
Fair Value Measurement [Line Items]
Derivative liabilitly		77,000
Fair Value, Measurements, Recurring [Member] | Equity Contract [Member] | Level 2 [Member]
Fair Value Measurement [Line Items]
Derivative liabilitly		0
Fair Value, Measurements, Recurring [Member] | Equity Contract [Member] | Level 3 [Member]
Fair Value Measurement [Line Items]
Derivative liabilitly		$ 0

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Fair Value Measurement [Line Items]

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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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).

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Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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