Reserves for Unpaid Losses and Loss Adjustment Expenses (Average Annual Percentage Payout of Incurred Claims) (Details)	Dec. 31, 2017	[1]
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	30.50%
Average annual percentage payout of incurred claims, year two	23.90%
Average annual percentage payout of incurred claims, year three	19.00%
Average annual percentage payout of incurred claims, year four	13.10%
Average annual percentage payout of incurred claims, year five	6.60%
Average annual percentage payout of incurred claims, year six	1.60%
Average annual percentage payout of incurred claims, year seven	1.50%
Average annual percentage payout of incurred claims, year eight	1.80%
Average annual percentage payout of incurred claims, year nine	0.70%
Average annual percentage payout of incurred claims, year ten	1.30%
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	45.10%
Average annual percentage payout of incurred claims, year two	23.70%
Average annual percentage payout of incurred claims, year three	8.10%
Average annual percentage payout of incurred claims, year four	6.50%
Average annual percentage payout of incurred claims, year five	4.70%
Average annual percentage payout of incurred claims, year six	3.70%
Average annual percentage payout of incurred claims, year seven	1.50%
Average annual percentage payout of incurred claims, year eight	1.80%
Average annual percentage payout of incurred claims, year nine	1.00%
Average annual percentage payout of incurred claims, year ten	1.70%
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	52.00%
Average annual percentage payout of incurred claims, year two	28.60%
Average annual percentage payout of incurred claims, year three	10.80%
Average annual percentage payout of incurred claims, year four	5.60%
Average annual percentage payout of incurred claims, year five	1.30%
Average annual percentage payout of incurred claims, year six	0.70%
Average annual percentage payout of incurred claims, year seven	0.30%
Average annual percentage payout of incurred claims, year eight	0.20%
Average annual percentage payout of incurred claims, year nine	0.10%
Average annual percentage payout of incurred claims, year ten	0.40%

[1] The average annual percentage payout is calculated from a paid losses and ALAE development pattern based on an actuarial analysis of the paid losses and ALAE movements by accident year for each disaggregation category. The paid losses and ALAE development pattern provides the expected percentage of ultimate losses and ALAE to be paid in each year. The pattern considers all accident years included in the claims development tables