Investments - Allowance for Credit Losses (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Period	$ 3,300,000	$ 0	$ 0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	1,300,000		4,600,000
Reduction Due to Sales	(300,000)
End of the Period	4,600,000	4,600,000	3,300,000
Net Increase (Decrease) in Allowance on Previously Impaired Securities	500,000
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	(200,000)
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Period	300,000	0	0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	0	1,100,000
End of the Period	300,000	1,100,000	300,000
Net Increase (Decrease) in Allowance on Previously Impaired Securities	200,000
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	(200,000)
Bonds and Notes
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Period	3,000,000.0	0	0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	1,300,000	3,500,000
Reduction Due to Sales	(300,000)
End of the Period	4,300,000	$ 3,500,000	$ 3,000,000.0
Net Increase (Decrease) in Allowance on Previously Impaired Securities	300,000
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	$ 0

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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Balance Type:	credit
Period Type:	instant

- 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

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

+ References

No definition available.

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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.

+ References

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- Definition

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).

+ References

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