Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross reserve for loss and loss expense, at beginning of year	$ 4,260,355	$ 4,067,163
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year	554,269	547,066
Net reserve for loss and loss expense, at beginning of year	3,706,086	3,520,097
Incurred loss and loss expense for claims occurring in the current year	886,801	825,201
Total incurred loss and loss expense	835,024	804,273
Paid loss and loss expense for claims occurring in the current year	227,505	221,422
Paid loss and loss expense for claims occurring in the prior years	455,919	479,736
Total paid loss and loss expense	683,424	701,158
Net reserve for loss and loss expense, at end of period	3,857,686	3,623,212
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period	579,567	553,694
Gross reserve for loss and loss expense, at end of period	4,437,253	4,176,906
Allowance for credit losses	(1,777)	(2,396)	$ (1,840)	$ (1,777)	$ (1,502)	$ (4,400)
Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development	(51,777)	(20,928)
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development	(52,000)	(25,000)
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | Workers' Compensation Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development	(20,000)	(25,000)
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | General Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development	(25,000)	(10,000)
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | Commercial Automobile [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development		10,000
Property Insurance [Member] | Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development		$ 4,100
E&S Lines | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (7,000)
Reinsurance Recoverable [Domain]
Liability for Claims and Claims Adjustment Expense [Line Items]
Allowance for credit losses			(1,840)	(1,777)	(1,502)	(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	$ 0	$ 0	$ 2,903	[1]

[1] Represents the impact of our adoption of ASU 2016-13, Financial Instruments - Credit Losses.