Reserves for Unpaid Losses and Loss Adjustment Expenses (Average Annual Percentage Payout of Incurred Claims) (Details)	Dec. 31, 2019
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	14.50%
Average annual percentage payout of incurred claims, year two	25.60%
Average annual percentage payout of incurred claims, year three	26.30%
Average annual percentage payout of incurred claims, year four	16.30%
Average annual percentage payout of incurred claims, year five	10.00%
Average annual percentage payout of incurred claims, year six	3.60%
Average annual percentage payout of incurred claims, year seven	2.60%
Average annual percentage payout of incurred claims, year eight	0.10%
Average annual percentage payout of incurred claims, year ten	1.00%
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	12.40%
Average annual percentage payout of incurred claims, year two	15.80%
Average annual percentage payout of incurred claims, year three	19.90%
Average annual percentage payout of incurred claims, year four	21.30%
Average annual percentage payout of incurred claims, year five	14.30%
Average annual percentage payout of incurred claims, year six	4.70%
Average annual percentage payout of incurred claims, year seven	2.20%
Average annual percentage payout of incurred claims, year eight	7.60%
Average annual percentage payout of incurred claims, year nine	0.80%
Average annual percentage payout of incurred claims, year ten	1.00%
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	36.80%
Average annual percentage payout of incurred claims, year two	16.80%
Average annual percentage payout of incurred claims, year three	5.80%
Average annual percentage payout of incurred claims, year four	6.30%
Average annual percentage payout of incurred claims, year five	5.80%
Average annual percentage payout of incurred claims, year six	7.00%
Average annual percentage payout of incurred claims, year seven	5.70%
Average annual percentage payout of incurred claims, year eight	2.80%
Average annual percentage payout of incurred claims, year nine	6.90%
Average annual percentage payout of incurred claims, year ten	6.10%
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	63.50%
Average annual percentage payout of incurred claims, year two	22.70%
Average annual percentage payout of incurred claims, year three	6.20%
Average annual percentage payout of incurred claims, year four	3.30%
Average annual percentage payout of incurred claims, year five	2.80%
Average annual percentage payout of incurred claims, year six	1.50%
Average annual percentage payout of incurred claims, year seven	0.50%
Average annual percentage payout of incurred claims, year eight	1.60%
Average annual percentage payout of incurred claims, year nine	0.10%
Average annual percentage payout of incurred claims, year ten	(2.20%)
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	31.90%
Average annual percentage payout of incurred claims, year two	37.80%
Average annual percentage payout of incurred claims, year three	17.60%
Average annual percentage payout of incurred claims, year four	6.90%
Average annual percentage payout of incurred claims, year five	4.20%
Average annual percentage payout of incurred claims, year six	1.40%
Average annual percentage payout of incurred claims, year seven	1.50%
Average annual percentage payout of incurred claims, year eight	(1.30%)
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	73.20%
Average annual percentage payout of incurred claims, year two	20.30%
Average annual percentage payout of incurred claims, year three	6.50%
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	77.20%
Average annual percentage payout of incurred claims, year two	15.80%
Average annual percentage payout of incurred claims, year three	1.70%
Average annual percentage payout of incurred claims, year four	0.50%
Average annual percentage payout of incurred claims, year five	3.60%
Average annual percentage payout of incurred claims, year six	1.20%