Losses and Loss Adjustment Expenses - Reconciliation Of Net Incurred Losses Prior To Provision For Bad Debt (Details) - Non-life Run-off - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	$ 6,656,097
Provision for Bad Debt	51,040
Total Net Reserves	6,707,137	$ 6,288,330
Asbestos
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	1,740,810
Provision for Bad Debt	21,289
Total Net Reserves	1,762,099	1,493,110
Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	308,613
Provision for Bad Debt	7,616
Total Net Reserves	316,229	210,479
General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	813,541
Provision for Bad Debt	7,281
Total Net Reserves	820,822	714,709
Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	1,710,528
Provision for Bad Debt	4,124
Total Net Reserves	1,714,652	1,652,898
Marine, aviation and transit
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	342,989
Provision for Bad Debt	1,757
Total Net Reserves	344,746	306,017
Construction defect
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	124,132
Provision for Bad Debt	1
Total Net Reserves	124,133	114,046
Professional indemnity/Directors & Officers
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	655,260
Provision for Bad Debt	1,144
Total Net Reserves	656,404	592,918
Motor
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	480,599
Provision for Bad Debt	2,773
Total Net Reserves	483,372	719,721
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	178,215
Provision for Bad Debt	245
Total Net Reserves	178,460	197,690
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	301,410
Provision for Bad Debt	4,810
Total Net Reserves	$ 306,220	$ 286,742