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Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 15.0	$ 10.0
Increase (decrease) in estimates for claims	22.0	(16.0)
Adjustment expense	3.0	4.0
Liability recognized for best estimate net of reinsurance	303.0
Liability recognized for best estimate net of reinsurance, reported losses	292.0
Liability recognized for best estimate net of reinsurance, incurred but not reported (IBNR)	11.0
Liability recognized for best estimate net of reinsurance, current accident year losses	3.0
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability recognized for best estimate net of reinsurance	255.0
Liability recognized for best estimate net of reinsurance, current accident year losses	1.0
Favorable reserve development net of premium offsets	3.0	12.0
Reinsurance & Monoline Excess
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability recognized for best estimate net of reinsurance	48.0
Liability recognized for best estimate net of reinsurance, current accident year losses	2.0
Unfavorable reserve development net of premium offsets	$ 0.3	$ 8.0

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

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- Definition

Current year monetary amount of the change in worker's compensation reserve liability resulting from the discount applied to reduce the reserve to present value.

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Favorable Reserve Development Net Of Premium Offsets

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- Definition

Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance, Current Accident Year Losses

+ References

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- Definition

Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance, Incurred but Not Reported (IBNR) Claims, Amount

+ References

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- Definition

Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance, Reported Claims, Amount

+ References

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- Definition

Liability For Unpaid Claims And Claims Adjustment Expense Incurred Claims Undiscounted Basis Prior Years, Increase (Decrease)

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- Definition

Liability for Claims and Claims Adjustment Expense, Net Of Reinsurance

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Unfavorable Reserve Development Net Of Premium Offsets

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