Reserves for Unpaid Losses and Loss Adjustment Expenses (Average Annual Percentage Payout of Incurred Claims) (Details)	Dec. 31, 2018
Specialty Commercial 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	27.10%
Average annual percentage payout of incurred claims, year two	24.00%
Average annual percentage payout of incurred claims, year three	20.50%
Average annual percentage payout of incurred claims, year four	13.60%
Average annual percentage payout of incurred claims, year five	7.90%
Average annual percentage payout of incurred claims, year six	2.70%
Average annual percentage payout of incurred claims, year seven	1.70%
Average annual percentage payout of incurred claims, year eight	1.80%
Average annual percentage payout of incurred claims, year nine	0.40%
Average annual percentage payout of incurred claims, year ten	0.30%
Standard Commercial 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	45.30%
Average annual percentage payout of incurred claims, year two	25.20%
Average annual percentage payout of incurred claims, year three	9.20%
Average annual percentage payout of incurred claims, year four	6.90%
Average annual percentage payout of incurred claims, year five	4.80%
Average annual percentage payout of incurred claims, year six	3.70%
Average annual percentage payout of incurred claims, year seven	1.30%
Average annual percentage payout of incurred claims, year eight	1.00%
Average annual percentage payout of incurred claims, year nine	1.40%
Average annual percentage payout of incurred claims, year ten	1.20%
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	53.70%
Average annual percentage payout of incurred claims, year two	28.00%
Average annual percentage payout of incurred claims, year three	10.20%
Average annual percentage payout of incurred claims, year four	5.40%
Average annual percentage payout of incurred claims, year five	1.50%
Average annual percentage payout of incurred claims, year six	0.30%
Average annual percentage payout of incurred claims, year seven	0.30%
Average annual percentage payout of incurred claims, year eight	0.30%
Average annual percentage payout of incurred claims, year nine	0.10%
Average annual percentage payout of incurred claims, year ten	0.20%