Debt Securities, Available-for-sale, Allowance for Credit Losses Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0.0	$ 0.0
All Other Corporate Bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0	$ 6.8
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not Previously Recorded		0.0
Change in allowance due to change in intent to hold securities to maturity		0.5
Debt Securities, Available-for-sale, Allowance for Credit Loss, Securities Sold	(7.3)
Public Utilities
Debt Securities, Available-for-sale, Allowance for Credit Loss
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	$ 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not Previously Recorded	4.6
Change in allowance due to change in intent to hold securities to maturity	0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Securities Sold	$ (4.6)

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 https://asc.fasb.org/extlink&oid=124269663&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 https://asc.fasb.org/extlink&oid=124269663&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 https://asc.fasb.org/extlink&oid=126970911&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 https://asc.fasb.org/extlink&oid=124258926&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.

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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.

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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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