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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, 2019 | Dec. 31, 2018
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Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | $ 3,093,275,000 | $ 2,985,161,000
Equity securities (cost $648,282; $544,082) | 724,751,000 | 529,631,000
Short-term investments (cost $494,060; $254,518) | 494,135,000 | 253,299,000
Total Return Swap [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities fair value | 0 | 4,851,000
Equity Contract [Member] |  |
Fair Value Measurement [Line Items] |  |
Derivative liabilitly | 77,000 | 3,000
Fair Value, Measurements, Recurring [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 3,093,275,000 | 2,985,161,000
Equity securities (cost $648,282; $544,082) | 724,751,000 | 529,631,000
Short-term investments (cost $494,060; $254,518) | 494,135,000 | 253,299,000
Other assets: | 5,665,000 | 5,557,000
Total assets at fair value | 4,317,826,000 | 3,773,648,000
Liabilities fair value | 77,000 | 4,854,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 22,637,000 | 25,003,000
Equity securities (cost $648,282; $544,082) | 586,367,000 | 430,973,000
Short-term investments (cost $494,060; $254,518) | 464,055,000 | 236,515,000
Other assets: | 0 | 0
Total assets at fair value | 1,073,059,000 | 692,491,000
Liabilities fair value | 77,000 | 3,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 3,070,638,000 | 2,960,158,000
Equity securities (cost $648,282; $544,082) | 49,708,000 | 31,433,000
Short-term investments (cost $494,060; $254,518) | 30,080,000 | 16,784,000
Other assets: | 5,665,000 | 5,557,000
Total assets at fair value | 3,156,091,000 | 3,013,932,000
Liabilities fair value | 0 | 4,851,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 0 | 0
Equity securities (cost $648,282; $544,082) | 1,203,000 | 1,445,000
Short-term investments (cost $494,060; $254,518) | 0 | 0
Other assets: | 0 | 0
Total assets at fair value | 1,203,000 | 1,445,000
Liabilities fair value | 0 | 0
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 22,637,000 | 25,003,000
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 22,637,000 | 25,003,000
Fair Value, Measurements, Recurring [Member] | US Government Agencies Debt Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 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 $2,973,276; $2,969,541) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 2,554,208,000 | 2,620,132,000
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 2,554,208,000 | 2,620,132,000
Fair Value, Measurements, Recurring [Member] | Municipal Securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 63,003,000 | 30,952,000
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 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 $2,973,276; $2,969,541) | 63,003,000 | 30,952,000
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 235,565,000 | 105,524,000
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 235,565,000 | 105,524,000
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 199,217,000 | 165,789,000
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 199,217,000 | 165,789,000
Fair Value, Measurements, Recurring [Member] | Collateralized Loan Obligations [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 18,645,000 | 37,761,000
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 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 $2,973,276; $2,969,541) | 18,645,000 | 37,761,000
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed Securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fixed maturity securities (amortized cost $2,973,276; $2,969,541) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 586,367,000 | 430,973,000
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 586,367,000 | 430,973,000
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 49,708,000 | 31,433,000
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 49,708,000 | 31,433,000
Fair Value, Measurements, Recurring [Member] | Nonredeemable Preferred Stock [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 1,203,000 | 1,445,000
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 1,203,000 | 1,445,000
Fair Value, Measurements, Recurring [Member] | Private Equity Funds Net Asset Value [Member] |  |
Fair Value Measurement [Line Items] |  |
Equity securities (cost $648,282; $544,082) | 87,473,000 | 65,780,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] |  |
Fair Value Measurement [Line Items] |  |
Short-term investments (cost $494,060; $254,518) | 32,902,000 | 48,256,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Short-term investments (cost $494,060; $254,518) | 2,822,000 | 31,472,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Short-term investments (cost $494,060; $254,518) | 30,080,000 | 16,784,000
Fair Value, Measurements, Recurring [Member] | Short-term Debt [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Short-term investments (cost $494,060; $254,518) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] |  |
Fair Value Measurement [Line Items] |  |
Short-term investments (cost $494,060; $254,518) | 461,233,000 | 205,043,000
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Short-term investments (cost $494,060; $254,518) | 461,233,000 | 205,043,000
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Short-term investments (cost $494,060; $254,518) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Short-term investments (cost $494,060; $254,518) | 0 | 0
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] |  |
Fair Value Measurement [Line Items] |  |
Derivative liabilitly |  | 4,851,000
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Derivative liabilitly |  | 0
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Derivative liabilitly |  | 4,851,000
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Derivative liabilitly |  | 0
Fair Value, Measurements, Recurring [Member] | Equity Contract [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Derivative liabilitly | 77,000 | 3,000
Fair Value, Measurements, Recurring [Member] | Equity Contract [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Derivative liabilitly | 0 | 0
Fair Value, Measurements, Recurring [Member] | Equity Contract [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Derivative liabilitly | $ 0 | $ 0

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 35 -Paragraph 54B -URI http://asc.fasb.org/extlink&oid=99392525&loc=SL7495116-110257 + Details
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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313 + Details
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