Reserves for Losses and Loss Adjustment Expenses (Average Annual Percentage Payout of Incurred Claims) (Details)	Dec. 31, 2021
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	15.30%
Average annual percentage payout of incurred claims, year two	25.70%
Average annual percentage payout of incurred claims, year three	25.50%
Average annual percentage payout of incurred claims, year four	16.60%
Average annual percentage payout of incurred claims, year five	9.00%
Average annual percentage payout of incurred claims, year six	4.50%
Average annual percentage payout of incurred claims, year seven	2.30%
Average annual percentage payout of incurred claims, year eight	0.80%
Average annual percentage payout of incurred claims, year nine	0.30%
Average annual percentage payout of incurred claims, year ten	0.00%
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	9.40%
Average annual percentage payout of incurred claims, year two	17.80%
Average annual percentage payout of incurred claims, year three	20.50%
Average annual percentage payout of incurred claims, year four	23.10%
Average annual percentage payout of incurred claims, year five	14.90%
Average annual percentage payout of incurred claims, year six	4.90%
Average annual percentage payout of incurred claims, year seven	3.60%
Average annual percentage payout of incurred claims, year eight	3.00%
Average annual percentage payout of incurred claims, year nine	2.00%
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	41.00%
Average annual percentage payout of incurred claims, year two	23.80%
Average annual percentage payout of incurred claims, year three	10.20%
Average annual percentage payout of incurred claims, year four	8.20%
Average annual percentage payout of incurred claims, year five	6.00%
Average annual percentage payout of incurred claims, year six	3.80%
Average annual percentage payout of incurred claims, year seven	2.50%
Average annual percentage payout of incurred claims, year eight	2.00%
Average annual percentage payout of incurred claims, year nine	1.50%
Average annual percentage payout of incurred claims, year ten	1.00%
General Aviation and Satellite Component [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	77.90%
Average annual percentage payout of incurred claims, year two	8.30%
Average annual percentage payout of incurred claims, year three	2.70%
Average annual percentage payout of incurred claims, year four	4.50%
Average annual percentage payout of incurred claims, year five	3.00%
Average annual percentage payout of incurred claims, year six	1.60%
Average annual percentage payout of incurred claims, year seven	1.10%
Average annual percentage payout of incurred claims, year eight	0.70%
Average annual percentage payout of incurred claims, year nine	0.20%
Average annual percentage payout of incurred claims, year ten	0.00%
Workers Compensation 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	33.50%
Average annual percentage payout of incurred claims, year two	31.10%
Average annual percentage payout of incurred claims, year three	16.10%
Average annual percentage payout of incurred claims, year four	9.00%
Average annual percentage payout of incurred claims, year five	3.30%
Average annual percentage payout of incurred claims, year six	1.90%
Average annual percentage payout of incurred claims, year seven	1.50%
Average annual percentage payout of incurred claims, year eight	1.30%
Average annual percentage payout of incurred claims, year nine	1.20%
Average annual percentage payout of incurred claims, year ten	1.10%
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	44.60%
Average annual percentage payout of incurred claims, year two	36.90%
Average annual percentage payout of incurred claims, year three	13.90%
Average annual percentage payout of incurred claims, year four	4.60%
Average annual percentage payout of incurred claims, year five	0.00%
Average annual percentage payout of incurred claims, year six	0.00%
Average annual percentage payout of incurred claims, year seven	0.00%
Average annual percentage payout of incurred claims, year eight	0.00%
Average annual percentage payout of incurred claims, year nine	0.00%
Average annual percentage payout of incurred claims, year ten	0.00%
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	62.10%
Average annual percentage payout of incurred claims, year two	25.50%
Average annual percentage payout of incurred claims, year three	7.40%
Average annual percentage payout of incurred claims, year four	3.70%
Average annual percentage payout of incurred claims, year five	0.90%
Average annual percentage payout of incurred claims, year six	0.40%
Average annual percentage payout of incurred claims, year seven	0.00%
Average annual percentage payout of incurred claims, year eight	0.00%
Average annual percentage payout of incurred claims, year nine	0.00%
Average annual percentage payout of incurred claims, year ten	0.00%
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	64.70%
Average annual percentage payout of incurred claims, year two	27.50%
Average annual percentage payout of incurred claims, year three	4.10%
Average annual percentage payout of incurred claims, year four	2.10%
Average annual percentage payout of incurred claims, year five	1.00%
Average annual percentage payout of incurred claims, year six	0.50%
Average annual percentage payout of incurred claims, year seven	0.10%
Average annual percentage payout of incurred claims, year eight	0.00%
Average annual percentage payout of incurred claims, year nine	0.00%
Average annual percentage payout of incurred claims, year ten	0.00%