Reserves for Losses and Loss Adjustment Expenses (Average Annual Percentage Payout of Incurred Claims) (Details)	Dec. 31, 2020
Commercial Auto business units [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Average annual percentage payout of incurred claims, year one	16.40%
Average annual percentage payout of incurred claims, year two	24.60%
Average annual percentage payout of incurred claims, year three	25.50%
Average annual percentage payout of incurred claims, year four	15.90%
Average annual percentage payout of incurred claims, year five	9.60%
Average annual percentage payout of incurred claims, year six	4.80%
Average annual percentage payout of incurred claims, year seven	1.60%
Average annual percentage payout of incurred claims, year eight	1.00%
Average annual percentage payout of incurred claims, year nine	0.60%
E&S Casualty business unit [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Average annual percentage payout of incurred claims, year one	9.70%
Average annual percentage payout of incurred claims, year two	20.10%
Average annual percentage payout of incurred claims, year three	22.60%
Average annual percentage payout of incurred claims, year four	23.30%
Average annual percentage payout of incurred claims, year five	12.60%
Average annual percentage payout of incurred claims, year six	3.20%
Average annual percentage payout of incurred claims, year seven	3.00%
Average annual percentage payout of incurred claims, year eight	2.80%
Average annual percentage payout of incurred claims, year nine	1.90%
Average annual percentage payout of incurred claims, year ten	0.80%
Commercial Accounts business unit [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Average annual percentage payout of incurred claims, year one	44.80%
Average annual percentage payout of incurred claims, year two	26.30%
Average annual percentage payout of incurred claims, year three	7.70%
Average annual percentage payout of incurred claims, year four	7.10%
Average annual percentage payout of incurred claims, year five	3.70%
Average annual percentage payout of incurred claims, year six	3.10%
Average annual percentage payout of incurred claims, year seven	2.60%
Average annual percentage payout of incurred claims, year eight	2.10%
Average annual percentage payout of incurred claims, year nine	1.60%
Average annual percentage payout of incurred claims, year ten	1.00%
Aviation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Average annual percentage payout of incurred claims, year one	64.50%
Average annual percentage payout of incurred claims, year two	23.70%
Average annual percentage payout of incurred claims, year three	5.00%
Average annual percentage payout of incurred claims, year four	3.80%
Average annual percentage payout of incurred claims, year five	3.20%
Average annual percentage payout of incurred claims, year six	1.20%
Average annual percentage payout of incurred claims, year seven	0.60%
Average annual percentage payout of incurred claims, year eight	(2.00%)
Runoff [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Average annual percentage payout of incurred claims, year one	38.10%
Average annual percentage payout of incurred claims, year two	27.90%
Average annual percentage payout of incurred claims, year three	16.30%
Average annual percentage payout of incurred claims, year four	8.50%
Average annual percentage payout of incurred claims, year five	2.40%
Average annual percentage payout of incurred claims, year six	1.90%
Average annual percentage payout of incurred claims, year seven	1.40%
Average annual percentage payout of incurred claims, year eight	1.40%
Average annual percentage payout of incurred claims, year nine	1.20%
Average annual percentage payout of incurred claims, year ten	0.90%
Specialty Programs Business Unit [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Average annual percentage payout of incurred claims, year one	58.50%
Average annual percentage payout of incurred claims, year two	35.80%
Average annual percentage payout of incurred claims, year three	5.70%
Personal Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Average annual percentage payout of incurred claims, year one	56.00%
Average annual percentage payout of incurred claims, year two	28.80%
Average annual percentage payout of incurred claims, year three	8.90%
Average annual percentage payout of incurred claims, year four	4.70%
Average annual percentage payout of incurred claims, year five	0.90%
Average annual percentage payout of incurred claims, year six	0.30%
Average annual percentage payout of incurred claims, year seven	0.10%
Average annual percentage payout of incurred claims, year eight	0.20%
Average annual percentage payout of incurred claims, year nine	0.10%
Property [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Average annual percentage payout of incurred claims, year one	67.50%
Average annual percentage payout of incurred claims, year two	25.00%
Average annual percentage payout of incurred claims, year three	3.60%
Average annual percentage payout of incurred claims, year four	2.20%
Average annual percentage payout of incurred claims, year five	1.10%
Average annual percentage payout of incurred claims, year six	0.60%