Investments - Allowance for Credit Losses (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for Credit Losses at Beginning of the Year	$ 5,400,000	$ 0
Impact of Adopting ASU 2016-13		0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	200,000	5,600,000
Reduction Due to Sales	(500,000)	(500,000)
Net Increase (Decrease) in Allowance on Previously Impaired Securities	0	0
Write-offs Charged Against Allowance	(300,000)	(300,000)
Allowance for Credit Losses at End of Period	4,800,000	4,800,000
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for Credit Losses at Beginning of the Year	600,000	0
Impact of Adopting ASU 2016-13		0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	0	600,000
Reduction Due to Sales	(100,000)	(100,000)
Net Increase (Decrease) in Allowance on Previously Impaired Securities	100,000	100,000
Write-offs Charged Against Allowance	0	0
Allowance for Credit Losses at End of Period	600,000	600,000
Bonds and Notes
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for Credit Losses at Beginning of the Year	4,800,000	0
Impact of Adopting ASU 2016-13		0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	200,000	5,000,000.0
Reduction Due to Sales	(400,000)	(400,000)
Net Increase (Decrease) in Allowance on Previously Impaired Securities	(100,000)	(100,000)
Write-offs Charged Against Allowance	(300,000)	(300,000)
Allowance for Credit Losses at End of Period	$ 4,200,000	$ 4,200,000

Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

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- Definition

Amount of expense for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), for which no credit loss was previously recorded.

+ References

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- Definition

Amount of expense (reversal of expense) for allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), not intended to be sold and not more likely than not will be required to sell before recovery of amortized cost basis, with previously recorded allowance.

+ References

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- Definition

Amount of increase (decrease) in allowance for credit loss of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of decrease in allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) from sale.

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Amount of writeoff of allowance for credit loss on 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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