Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves			$ 336,942	$ 389,740
Net losses and loss adjustment expenses	$ 900,230	$ 860,986	2,454,912	2,340,868
Reinsurance recoverables	$ 94,400	$ 171,100	443,800	394,900
Gen liab, workers comp and professional liab lines Insurance segment, and aviation, auto and whole account Reinsurance segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves			(257,700)
Gen liab, workers comp, marine & energy and prof liab Insurance segment and credit and surety and marine and energy Reinsurance segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves				(312,200)
2019 Catastrophes [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net losses and loss adjustment expenses, net of assumed reinstatement premiums			42,600
Net losses and loss adjustment expenses			45,100
Net assumed reinstatement premiums			(2,500)
Reinsurance recoverables			$ 4,800
2018 Catastrophes [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net losses and loss adjustment expenses, net of assumed reinstatement premiums				75,700
Net losses and loss adjustment expenses				77,800
Net assumed reinstatement premiums				(2,100)
Reinsurance recoverables				$ 28,600