Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident years	$ 1,266,996	$ 1,088,353	$ 3,420,906	$ 3,009,260
Favorable (adverse) development on prior years' loss reserves			204,411	366,171
2021 Catastrophes [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident years				182,300
Russia-Ukraine conflict
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident years			35,000
Ceded losses			70,000
Hurricane Ian
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident years			70,000
Ceded losses			100,000
Insurance [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident years	1,100,511	866,481	2,906,031	2,448,034
Insurance [Member] | Workers Comp, Property, Marine and Energy and Programs Insurance Segment [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves			155,000
Insurance [Member] | General Liab Property Workers Comp Prof Liability and Marine and Energy Insurance Segment [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves				324,800
Reinsurance [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident years	166,485	221,872	514,875	561,226
Reinsurance [Member] | Property and Credit and Surety Reinsurance Segment
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves			41,700
Reinsurance [Member] | Property Reinsurance Segment
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves				(30,600)
Program services and other fronting [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ceded losses	$ (1,300,000)	$ (620,000)	(2,300,000)	$ (1,700,000)
Program services and other fronting [Member] | Hurricane Ian
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident years			850,000
Ceded losses			$ 850,000