Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross reserve for loss and loss expense, at beginning of year			$ 4,067,163		$ 3,893,868
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year			547,066	[1]	537,388
Net reserve for loss and loss expense, at beginning of year			3,520,097		3,356,480
Incurred loss and loss expense for claims occurring in the current year			1,295,285		1,200,878
Total incurred loss and loss expense			1,252,075		1,166,238
Paid loss and loss expense for claims occurring in the current year			421,981		384,436
Paid loss and loss expense for claims occurring in the prior years			646,974		631,589
Total paid loss and loss expense			1,068,955		1,016,025
Net reserve for loss and loss expense, at end of period	$ 3,703,217	$ 3,506,693	3,703,217		3,506,693
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period	590,059	548,938	590,059		548,938
Gross reserve for loss and loss expense, at end of period	4,293,276	4,055,631	4,293,276		4,055,631
Loss Reserve for COVID-19			10,000
Loss from Catastrophes	79,500	$ 24,200	195,900		74,500
Allowance for credit losses	$ (1,831)		(1,831)			$ (2,396)	$ (4,400)
Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development			(43,210)		(34,640)
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development			(50,000)		(41,000)
Property Insurance [Member] | Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development			6,800		6,400
Workers' Compensation Insurance [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development			(40,000)		(33,000)
General Liability [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development			(20,000)		(10,000)
Personal Automobile [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development					$ 2,000
Commercial Automobile [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development			$ 10,000
Reinsurance Recoverable [Domain]
Liability for Claims and Claims Adjustment Expense [Line Items]
Allowance for credit losses						(2,396)	(1,497)
Reinsurance Recoverable [Domain] | Retained Earnings [Member] | Accounting Standards Update 2016-13 [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Allowance for credit losses						$ 0	$ 2,903	[1],[2]

[1] Includes an adjustment of $2.9 million related to our adoption of ASU 2016-13. Refer to Note 2. "Adoption of Accounting Pronouncements" for additional information.

[2] See Note 2. "Adoption of Accounting Pronouncements" for additional information regarding our adoption of ASU 2016-13.