Investments (Fair Value and Gross Pre-Tax Net Unrealized Loss of Securities By Length of Time) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
U.S. Government and Government Agencies [Member]
AFS Fair Value, Less than 12 Months	$ 126,898	$ 166,975
AFS Unrealized Losses, Less than 12 months	(1,428)	(13,658)
AFS Fair Value, 12 months or longer	99,831	16,011
AFS Unrealized Losses, 12 months or longer	(25,017)	(6,668)
AFS Fair Value	226,729	182,986
AFS Unrealized Losses	(26,445)	(20,326)
Foreign Government [Member]
AFS Fair Value, Less than 12 Months	1,528	5,573
AFS Unrealized Losses, Less than 12 months	(122)	(608)
AFS Fair Value, 12 months or longer	7,788	2,456
AFS Unrealized Losses, 12 months or longer	(1,685)	(699)
AFS Fair Value	9,316	8,029
AFS Unrealized Losses	(1,807)	(1,307)
Obligations of States and Political Subdivisions [Member]
AFS Fair Value, Less than 12 Months	307,144	681,795
AFS Unrealized Losses, Less than 12 months	(8,921)	(43,767)
AFS Fair Value, 12 months or longer	282,238	16,618
AFS Unrealized Losses, 12 months or longer	(47,636)	(4,234)
AFS Fair Value	589,382	698,413
AFS Unrealized Losses	(56,557)	(48,001)
Corporate Securities [Member]
AFS Fair Value, Less than 12 Months	808,733	1,889,492
AFS Unrealized Losses, Less than 12 months	(27,732)	(164,197)
AFS Fair Value, 12 months or longer	1,392,386	133,223
AFS Unrealized Losses, 12 months or longer	(200,824)	(32,612)
AFS Fair Value	2,201,119	2,022,715
AFS Unrealized Losses	(228,556)	(196,809)
Collateralized Loan Obligations and Other Asset Backed Securities [Member]
AFS Fair Value, Less than 12 Months	522,941	916,423
AFS Unrealized Losses, Less than 12 months	(13,560)	(69,155)
AFS Fair Value, 12 months or longer	1,014,419	411,283
AFS Unrealized Losses, 12 months or longer	(93,105)	(52,565)
AFS Fair Value	1,537,360	1,327,706
AFS Unrealized Losses	(106,665)	(121,720)
Residential Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 Months	609,952	887,229
AFS Unrealized Losses, Less than 12 months	(28,785)	(76,432)
AFS Fair Value, 12 months or longer	725,818	108,041
AFS Unrealized Losses, 12 months or longer	(124,389)	(22,833)
AFS Fair Value	1,335,770	995,270
AFS Unrealized Losses	(153,174)	(99,265)
Commercial Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 Months	182,481	512,953
AFS Unrealized Losses, Less than 12 months	(8,363)	(37,815)
AFS Fair Value, 12 months or longer	455,518	77,181
AFS Unrealized Losses, 12 months or longer	(53,913)	(11,945)
AFS Fair Value	637,999	590,134
AFS Unrealized Losses	(62,276)	(49,760)
Available-for-sale Securities [Member]
AFS Fair Value, Less than 12 Months	2,559,677	5,060,440
AFS Unrealized Losses, Less than 12 months	(88,911)	(405,632)
AFS Fair Value, 12 months or longer	3,977,998	764,813
AFS Unrealized Losses, 12 months or longer	(546,569)	(131,556)
AFS Fair Value	6,537,675	5,825,253
AFS Unrealized Losses	$ (635,480)	$ (537,188)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-7

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-4

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-4

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: