Reserves for Unpaid Losses and Loss Adjustment Expenses (Average Annual Percentage Payout of Incurred Claims) (Details)	Dec. 31, 2016
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	32.40%
Average annual percentage payout of incurred claims, year two	24.80%
Average annual percentage payout of incurred claims, year three	17.40%
Average annual percentage payout of incurred claims, year four	12.00%
Average annual percentage payout of incurred claims, year five	6.00%
Average annual percentage payout of incurred claims, year six	1.20%
Average annual percentage payout of incurred claims, year seven	1.20%
Average annual percentage payout of incurred claims, year eight	1.50%
Average annual percentage payout of incurred claims, year nine	0.50%
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	44.60%
Average annual percentage payout of incurred claims, year two	23.00%
Average annual percentage payout of incurred claims, year three	8.70%
Average annual percentage payout of incurred claims, year four	6.60%
Average annual percentage payout of incurred claims, year five	5.00%
Average annual percentage payout of incurred claims, year six	3.30%
Average annual percentage payout of incurred claims, year seven	1.30%
Average annual percentage payout of incurred claims, year eight	1.40%
Average annual percentage payout of incurred claims, year nine	1.30%
Average annual percentage payout of incurred claims, year ten	(1.40%)
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	51.50%
Average annual percentage payout of incurred claims, year two	29.20%
Average annual percentage payout of incurred claims, year three	10.90%
Average annual percentage payout of incurred claims, year four	4.30%
Average annual percentage payout of incurred claims, year five	1.40%
Average annual percentage payout of incurred claims, year six	0.80%
Average annual percentage payout of incurred claims, year seven	0.70%
Average annual percentage payout of incurred claims, year eight	(0.70%)
Average annual percentage payout of incurred claims, year nine	1.90%
Average annual percentage payout of incurred claims, year ten	0.00%